BUG: Fix not to reindex on non-Categorical groups (GH9049)
closes #9049.
closes #9344 

_self._was_factor_ is not appropriate to judge whether grouper is Categorical or not, because it can be "True" when we groupby indices (not columns). So, I added another flag _self._is_categorical_ to judge Categorical state.

Also, I added a GroupBy test for MultiIndexed data, which was failed before this fix.

According to 8d2c2a8, original `_was_factor` seems not meaning "Categorical" but "level based grouping". Probably, ea1186d confused the meaning of `_was_factor`. How about renaming `_was_factor` to `_grouping_by_level` and to use both this and `_is_categorical` ?
 maybe better to create a new flag maybe
_grouping_typ defaulting to None thrn set to a string (level or categorical)
it's internal so ok to change this 
 Ok, I'll try with this approach.
 Improved by using `_grouping_type` instead of two boolean flags.
 pls add a release note (bug fix section) referencing the original issue
 Added a comment and release note, and squashed changes.
 @ledmonster pls rebase this when you can and see if you can address the comments above.
 Rebased and refactored `Grouping` class for `Categorical` grouper.
- evaluate `_labels` and `_group_index` instead of using `_grouping_type` flag in `_make_labels`.
- stop to convert Categorical `self.grouper` to ndarray. I checked some logics using `self.grouper`, but it seems to work fine without converting `Categorical` one to `ndarray`. (by modifying L1948 a little)
- use `cache_readonly` instead of `property` for `groups` property.
- add tests for datetime Categorical grouper.

Any feedbacks are welcome, thank you.
 this looks pretty good - cleaned up s lot of older code

can u run s perf test (the vbench suite)
and report of anythjng anomalous comes up
 I tried `./test_perf.sh -b master -t HEAD`, but it took too much machine resources. So, I'll try again tonight while I'm in bed, with `-r groupby` option.

BTW, I got some problems on running perf test, and made Pull #9332 for them.
 My Air Mac freezes by running `./test_perf.sh -b master -t HEAD -r groupby`, so I'll try it on EC2 instance later.
 @jreback

Finally, I could run performance test on my AIr Mac. (I don't know what was wrong before .. :sweat:)

It seems that this pull request doesn't break current performance. I'll try to write a performance test, which represents an effect of this pull request.

BTW, some of tests (`groupby_nth_float64_any` and `groupby_nth_float32_any`) filed with AssertionError, even in master branch.

### details

environment:
- python 2.7.8
- numpy 1.8.1

command:

```
./test_perf.sh -b master -t HEAD -r groupby
```

vb_suite.log

```
Invoked with :
--ncalls: 3
--repeats: 3


-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_ngroups_100_cumprod                  |  20.3053 |  28.8743 |   0.7032 |
groupby_ngroups_100_last                     |   0.6303 |   0.8399 |   0.7504 |
groupby_ngroups_100_min                      |   0.6464 |   0.7593 |   0.8513 |
groupby_ngroups_100_any                      |  14.2833 |  16.3154 |   0.8755 |
groupby_ngroups_100_head                     |   0.9576 |   1.0840 |   0.8834 |
groupby_int_count                            |   5.5679 |   6.1113 |   0.9111 |
groupby_ngroups_10000_var                    |   2.8907 |   3.1310 |   0.9232 |
groupby_last_float64                         |   4.9033 |   5.3020 |   0.9248 |
groupby_multi_size                           |  30.7086 |  33.1620 |   0.9260 |
groupby_ngroups_10000_last                   |   3.0739 |   3.2957 |   0.9327 |
groupby_ngroups_10000_mad                    | 6308.1783 | 6728.5376 |   0.9375 |
groupby_frame_singlekey_integer              |   3.1730 |   3.3813 |   0.9384 |
groupby_pivot_table                          |  26.1277 |  27.7747 |   0.9407 |
groupby_multi_different_functions            |  15.0627 |  15.9260 |   0.9458 |
groupby_ngroups_10000_value_counts           | 6888.7103 | 7216.8677 |   0.9545 |
groupby_ngroups_100_var                      |   0.4910 |   0.5110 |   0.9608 |
groupby_ngroups_100_mad                      |  64.2216 |  66.7377 |   0.9623 |
groupby_ngroups_10000_min                    |   2.9553 |   3.0704 |   0.9625 |
groupby_ngroups_100_all                      |  14.5303 |  14.9563 |   0.9715 |
groupby_ngroups_100_first                    |   0.6137 |   0.6301 |   0.9740 |
groupby_indices                              |  10.8583 |  11.1383 |   0.9749 |
groupby_first_float32                        |   4.5627 |   4.6653 |   0.9780 |
groupby_dt_size                              |  30.5503 |  31.2183 |   0.9786 |
groupby_ngroups_10000_any                    | 1387.1396 | 1417.0703 |   0.9789 |
groupby_series_nth_any                       |   5.7160 |   5.8363 |   0.9794 |
groupby_ngroups_10000_skew                   | 2316.1206 | 2363.0474 |   0.9801 |
groupby_ngroups_10000_cumsum                 | 1902.1820 | 1931.6096 |   0.9848 |
groupby_ngroups_100_std                      |   0.5527 |   0.5607 |   0.9857 |
groupby_ngroups_100_value_counts             |  70.4860 |  71.4906 |   0.9859 |
groupby_transform                            | 211.0896 | 214.0510 |   0.9862 |
groupby_series_simple_cython                 | 283.6780 | 287.3253 |   0.9873 |
groupby_ngroups_100_size                     |   0.6260 |   0.6333 |   0.9885 |
groupby_transform_ufunc                      | 170.6970 | 172.5584 |   0.9892 |
groupby_ngroups_10000_size                   |   5.5243 |   5.5800 |   0.9900 |
groupby_sum_booleans                         |   1.5647 |   1.5793 |   0.9907 |
groupby_multi_different_numpy_functions      |  16.0370 |  16.1840 |   0.9909 |
groupby_first_float64                        |   4.3423 |   4.3807 |   0.9912 |
groupby_ngroups_100_median                   |   0.5113 |   0.5156 |   0.9917 |
groupby_ngroups_10000_cummin                 | 1904.2930 | 1919.6667 |   0.9920 |
groupby_ngroups_100_cumsum                   |  20.0423 |  20.1993 |   0.9922 |
groupby_ngroups_10000_std                    |   2.9030 |   2.9247 |   0.9926 |
groupby_ngroups_100_mean                     |   0.4861 |   0.4890 |   0.9940 |
groupby_transform_series                     |  27.8977 |  28.0623 |   0.9941 |
groupby_ngroups_100_max                      |   0.6270 |   0.6304 |   0.9946 |
groupby_ngroups_100_describe                 | 226.6529 | 227.5487 |   0.9961 |
groupby_ngroups_10000_rank                   | 1884.1354 | 1885.8920 |   0.9991 |
groupby_ngroups_10000_max                    |   3.1803 |   3.1820 |   0.9995 |
groupby_dt_timegrouper_size                  |  26.5330 |  26.5294 |   1.0001 |
groupby_frame_cython_many_columns            |   4.1690 |   4.1670 |   1.0005 |
groupby_ngroups_10000_cumprod                | 1911.1340 | 1909.4703 |   1.0009 |
groupby_ngroups_10000_cummax                 | 1900.2120 | 1897.1416 |   1.0016 |
groupby_ngroups_10000_nunique                | 1431.4320 | 1427.3647 |   1.0028 |
groupby_nth_float64_none                     |  95.1933 |  94.8970 |   1.0031 |
groupby_ngroups_100_sem                      |   1.1113 |   1.1070 |   1.0039 |
groupby_first_object                         |  21.0333 |  20.9324 |   1.0048 |
groupby_multi_cython                         |  21.1920 |  21.0533 |   1.0066 |
groupby_series_nth_none                      |   1.7360 |   1.7220 |   1.0081 |
groupby_nth_object_none                      | 716.1887 | 709.5633 |   1.0093 |
groupby_ngroups_10000_cumcount               |  91.9007 |  90.9967 |   1.0099 |
groupby_agg_builtins2                        |  60.5311 |  59.9030 |   1.0105 |
groupby_ngroups_10000_pct_change             | 5491.0356 | 5427.6030 |   1.0117 |
groupby_ngroups_100_skew                     |  25.4494 |  25.0760 |   1.0149 |
groupby_ngroups_10000_prod                   |   3.0660 |   3.0204 |   1.0151 |
groupby_ngroups_10000_count                  |   2.9557 |   2.9027 |   1.0183 |
groupby_transform_multi_key3                 | 1221.3247 | 1198.5786 |   1.0190 |
groupby_ngroups_100_cumcount                 |   0.8957 |   0.8787 |   1.0194 |
groupby_transform_multi_key1                 | 114.1393 | 111.8406 |   1.0206 |
groupby_first_datetimes                      |  15.2536 |  14.9384 |   1.0211 |
groupby_frame_nth_none                       |   3.0247 |   2.9610 |   1.0215 |
groupby_ngroups_10000_all                    | 1471.4240 | 1439.2070 |   1.0224 |
groupby_ngroups_10000_tail                   | 105.2350 | 102.9210 |   1.0225 |
groupby_ngroups_10000_sum                    |   3.0403 |   2.9710 |   1.0233 |
groupby_int64_overflow                       | 482.9050 | 471.1819 |   1.0249 |
groupby_ngroups_10000_unique                 | 993.9707 | 969.5260 |   1.0252 |
groupby_frame_apply_overhead                 |  12.8290 |  12.5104 |   1.0255 |
groupby_apply_dict_return                    |  58.3300 |  56.8620 |   1.0258 |
groupby_ngroups_10000_diff                   | 1913.2260 | 1864.8180 |   1.0260 |
groupby_frame_apply                          |  58.9256 |  57.4137 |   1.0263 |
groupby_transform_multi_key4                 | 212.1150 | 206.3340 |   1.0280 |
groupby_ngroups_10000_describe               | 23349.1937 | 22703.3477 |   1.0284 |
groupby_nth_datetimes_any                    | 1401.2586 | 1361.6330 |   1.0291 |
groupby_ngroups_100_diff                     |  20.6420 |  20.0340 |   1.0303 |
groupby_last_datetimes                       |  15.2263 |  14.7664 |   1.0311 |
groupby_ngroups_100_unique                   |  10.5879 |  10.2584 |   1.0321 |
groupby_ngroups_100_tail                     |   1.1206 |   1.0823 |   1.0354 |
groupby_ngroups_10000_head                   | 101.8883 |  98.2103 |   1.0375 |
groupby_nth_datetimes_none                   | 690.9646 | 665.8187 |   1.0378 |
groupby_ngroups_100_rank                     |  21.0533 |  20.2037 |   1.0421 |
groupby_ngroups_100_pct_change               |  56.0367 |  53.7550 |   1.0424 |
groupby_ngroups_100_sum                      |   0.6863 |   0.6580 |   1.0431 |
groupby_ngroups_100_cummin                   |  21.7299 |  20.7803 |   1.0457 |
groupby_transform_multi_key2                 |  78.0807 |  74.6547 |   1.0459 |
groupby_last_object                          |  21.3246 |  20.3857 |   1.0461 |
groupby_transform_series2                    | 174.5074 | 166.2600 |   1.0496 |
groupby_multi_python                         | 206.9554 | 196.9930 |   1.0506 |
groupby_nth_float32_none                     | 117.9597 | 110.9044 |   1.0636 |
groupby_nth_object_any                       | 1407.9730 | 1323.5826 |   1.0638 |
groupby_multi_series_op                      |  19.2900 |  18.0667 |   1.0677 |
groupby_ngroups_100_cummax                   |  21.5353 |  20.1233 |   1.0702 |
groupby_ngroups_100_nunique                  |  15.7940 |  14.7266 |   1.0725 |
groupby_simple_compress_timing               |  43.8840 |  40.8143 |   1.0752 |
groupby_agg_builtins1                        |  15.7374 |  14.3619 |   1.0958 |
groupby_multi_count                          |  12.3970 |  11.0126 |   1.1257 |
groupby_frame_median                         |  10.1300 |   8.7897 |   1.1525 |
groupby_ngroups_100_count                    |   0.7610 |   0.6503 |   1.1702 |
groupby_last_float32                         |   5.0333 |   4.2017 |   1.1979 |
groupby_ngroups_10000_mean                   |   3.0813 |   2.5270 |   1.2194 |
groupby_frame_nth_any                        |   9.7840 |   7.9927 |   1.2241 |
groupby_ngroups_10000_median                 |   4.3104 |   3.3850 |   1.2734 |
groupby_ngroups_10000_first                  |   3.9930 |   2.9980 |   1.3319 |
groupby_ngroups_100_prod                     |   0.8580 |   0.6284 |   1.3654 |
groupby_ngroups_10000_sem                    |   6.3477 |   4.4421 |   1.4290 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [9f0842d] : ENH: Refactor groupby for Categorical grouper
Base   [9f439f0] : Merge pull request #9380 from behzadnouri/i8grby

bug in groupby when key space exceeds int64 bounds
```

Results are not so stable, probably because other applications are running on my Mac. For example, I ran `groupby_ngroups_10000_sem` again, and got following different result.

```
-------------------------------------------------------------------------------
groupby_ngroups_10000_sem                    |   4.0150 |   4.0263 |   0.9972 |
-------------------------------------------------------------------------------
```

`groupby_nth_float64_any` and `groupby_nth_float32_any` failed with a following traceback, which were recorded on benchmarks.db.

```
$ sqlite3 benchmarks.db
SQLite version 3.8.5 2014-08-15 22:37:57
Enter ".help" for usage hints.
sqlite> select * from results where checksum = '30b1995bdbe3973901ead3d9f96ff129' or checksum = '643e7456fe9d19192098079f8e257b0a';
30b1995bdbe3973901ead3d9f96ff129|5fd1fbd|2015-01-21 08:43:49.000000|||Traceback (most recent call last):
  File "/usr/local/opt/pyenv/versions/pandas27/lib/python2.7/site-packages/vbench/benchmark.py", line 90, in run
    repeat=self.repeat, force_ms=True)
  File "/usr/local/opt/pyenv/versions/pandas27/lib/python2.7/site-packages/vbench/benchmark.py", line 377, in magic_timeit
    best = min(timer.repeat(repeat, number)) / number
  File "/usr/local/opt/pyenv/versions/2.7.8/lib/python2.7/timeit.py", line 223, in repeat
    t = self.timeit(number)
  File "/usr/local/opt/pyenv/versions/2.7.8/lib/python2.7/timeit.py", line 195, in timeit
    timing = self.inner(it, self.timer)
  File "<magic-timeit>", line 6, in inner
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 894, in nth
    level=self.level, sort=self.sort)
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 2141, in _get_grouper
    level=level, sort=sort, in_axis=in_axis)
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 1870, in __init__
    self.grouper = _convert_grouper(index, grouper)
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 2168, in _convert_grouper
    raise AssertionError('Grouper and axis must be same length')
AssertionError: Grouper and axis must be same length

643e7456fe9d19192098079f8e257b0a|5fd1fbd|2015-01-21 08:43:49.000000|||Traceback (most recent call last):
  File "/usr/local/opt/pyenv/versions/pandas27/lib/python2.7/site-packages/vbench/benchmark.py", line 90, in run
    repeat=self.repeat, force_ms=True)
  File "/usr/local/opt/pyenv/versions/pandas27/lib/python2.7/site-packages/vbench/benchmark.py", line 377, in magic_timeit
    best = min(timer.repeat(repeat, number)) / number
  File "/usr/local/opt/pyenv/versions/2.7.8/lib/python2.7/timeit.py", line 223, in repeat
    t = self.timeit(number)
  File "/usr/local/opt/pyenv/versions/2.7.8/lib/python2.7/timeit.py", line 195, in timeit
    timing = self.inner(it, self.timer)
  File "<magic-timeit>", line 6, in inner
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 894, in nth
    level=self.level, sort=self.sort)
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 2141, in _get_grouper
    level=level, sort=sort, in_axis=in_axis)
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 1870, in __init__
    self.grouper = _convert_grouper(index, grouper)
  File "/private/var/folders/yh/7k_x_m811yvfffs2h44qw7cc0000gn/T/tmp1_vz7s/pandas/core/groupby.py", line 2168, in _convert_grouper
    raise AssertionError('Grouper and axis must be same length')
AssertionError: Grouper and axis must be same length
```
 Add new performance test for this issue.

A test result is like this. Performance improvement is not linear, so with larger DataFrame, performance ratio would be much smaller.

```

Invoked with :
--ncalls: 3
--repeats: 3


-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_sum_multiindex                       |   2.0807 |   4.8820 |   0.4262 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [9f0842d] : ENH: Refactor groupby for Categorical grouper
Base   [9f439f0] : Merge pull request #9380 from behzadnouri/i8grby

bug in groupby when key space exceeds int64 bounds
```
 can you incorporate at test by #9344 as well (in that the example both by_levels and by_columns should be the same). I think its reindexing in that case as well (and shouldn't).
thanks
 Okay, I'll try to add tests for #9344 on this weekend.
 Added a test for #9344. Now the test passes.
 Rebased.
 looks good. minor doc change. pls squash into 1-2 commits. and ping when ready.
 @jreback thank you for reviewing.

squashed to 2 commits (one is for bug fix, and the other is for refactoring), and rebased.
 thanks for this!
 My pleasure :smile_cat: 
