asv bench cleanup - groupby
Clean-up of the groupby benchmarks. 

This is an example of how they can be cleaned up (grouping benchmarks with the same setup in common classes, removing a lot of the setup functions in this way)

@jreback @qwhelan suppose this is OK to merge? 
This was one of the larger files, but as you can see, quite some line reduction is possible with some manual clean-up (also makes it clearer to add benchmarks)
 lgtm. I did sligthly different when I added some benchmarks [here](https://github.com/pydata/pandas/blob/master/asv_bench/benchmarks/binary_ops.py#L204)
as I subclassed. But both approaches make sense (and shorter boilerplate code)
 a couple fail: `asv dev --bench groupby`

```
[ 15.38%] Â·Â·Â· Running groupby.groupby_first_last.time_groupby_nth_float32_any                                                                                                                                                     fed
[ 15.38%] Â·Â·Â·Â·Â· Traceback (most recent call last):
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 774, in <module>
                    commands[mode](args)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 751, in main_run
                    result = benchmark.do_run()
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 443, in do_run
                    return self.run(*self._current_params)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 510, in run
                    timing = timer.timeit(number)
                  File "/Users/jreback/miniconda/lib/python2.7/timeit.py", line 201, in timeit
                    timing = self.inner(it, self.timer)
                  File "/Users/jreback/miniconda/lib/python2.7/timeit.py", line 100, in inner
                    _func()
                  File "/Users/jreback/pandas/asv_bench/benchmarks/groupby.py", line 64, in time_groupby_nth_float32_any
                    self.data2.groupby(self.labels).nth(0, dropna='all')
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 903, in nth
                    level=self.level, sort=self.sort)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 2158, in _get_grouper
                    level=level, sort=sort, in_axis=in_axis)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 1879, in __init__
                    self.grouper = _convert_grouper(index, grouper)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 2185, in _convert_grouper
                    raise AssertionError('Grouper and axis must be same length')
                AssertionError: Grouper and axis must be same length


[ 16.92%] Â·Â·Â· Running groupby.groupby_first_last.time_groupby_nth_float64_any                                                                                                                                                     fed
[ 16.92%] Â·Â·Â·Â·Â· Traceback (most recent call last):
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 774, in <module>
                    commands[mode](args)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 751, in main_run
                    result = benchmark.do_run()
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 443, in do_run
                    return self.run(*self._current_params)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/asv-0.2.dev795+c27e60f7-py2.7.egg/asv/benchmark.py", line 510, in run
                    timing = timer.timeit(number)
                  File "/Users/jreback/miniconda/lib/python2.7/timeit.py", line 201, in timeit
                    timing = self.inner(it, self.timer)
                  File "/Users/jreback/miniconda/lib/python2.7/timeit.py", line 100, in inner
                    _func()
                  File "/Users/jreback/pandas/asv_bench/benchmarks/groupby.py", line 70, in time_groupby_nth_float64_any
                    self.data.groupby(self.labels).nth(0, dropna='all')
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 903, in nth
                    level=self.level, sort=self.sort)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 2158, in _get_grouper
                    level=level, sort=sort, in_axis=in_axis)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 1879, in __init__
                    self.grouper = _convert_grouper(index, grouper)
                  File "/Users/jreback/miniconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 2185, in _convert_grouper
                    raise AssertionError('Grouper and axis must be same length')
                AssertionError: Grouper and axis must be same length
```
 yes, I know, but I didn't yet address that here (only restructuring the file, not fixing the tests themselves)

The subclassing is also fine, but there you had the same test function but different creation setup, so in that case it's more awkward to put it in one test class
 although, probably even the easier option is to use `params` in this case, as only the value for `N` changes, http://asv.readthedocs.org/en/latest/writing_benchmarks.html#parameterized-benchmarks
 oh, all these cool features. Ok, once the suite is cleaned up prob need to put a some docs in place when/how to do various things.
 It'll be a few hours until I can take a look (onsite + bunch of meetings
this evening), but I'm all for refactoring provided we prevent future
changes to the vbench groupby benchmarks (delete/comment out/scary warning).
On Sep 9, 2015 4:20 PM, "Jeff Reback" notifications@github.com wrote:

> oh, all these cool features. Ok, once the suite is cleaned up prob need to
> put a some docs in place when/how to do various things.
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/pydata/pandas/pull/10998#issuecomment-139070420.
 Yes, I think we should certainly ensure we don't change the vbenches anymore. Otherwise it has no point in starting to adapt the asv benches.

Will do a PR with a notice. I think we can leave them in for some time for those we didn't switch yet
 @jorisvandenbossche Other than the `xrange` call that's my fault, looks good to me.
 FYI, I have run benchmarks since 0.14 (not exactly on the tag as not a real easy way to do this). and pushed to gh-pages

http://pydata.github.io/pandas/
 Yeah, I've got some local patches to fix the run-against-tag issues that
result from trying to linearize history by using the first parent. I'll
chat with the asv devs to see if they're dead-set on that or whether that
can be configurable.
On Sep 12, 2015 1:13 PM, "Jeff Reback" notifications@github.com wrote:

> FYI, I have run benchmarks since 0.14 (not exactly on the tag as not a
> real easy way to do this). and pushed to gh-pages
> 
> http://pydata.github.io/pandas/
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/pydata/pandas/pull/10998#issuecomment-139815919.
 @jreback there seems to be some regressions since the last release (eg http://pydata.github.io/pandas/#plotting.plot_timeseries_period.time_plot_timeseries_period) maybe start listing them in the 0.17 release issue?
 @jreback @qwhelan another question: how do you handle the fact that some tests will fail on older pandas versions? (with vbench there was the start date you could give with the benchmark)
 @jorisvandenbossche I put that one on the 0.17.0 RLS issue. How do you 'find' regressions though (e.g. except for looking at charts directly). Did I not do something?
 Raising NotImplementedError is equivalent to SkipTest. Probably best done
in setup().
On Sep 12, 2015 4:56 PM, "Joris Van den Bossche" notifications@github.com
wrote:

> @jreback https://github.com/jreback @qwhelan
> https://github.com/qwhelan another question: how do you handle the fact
> that some tests will fail on older pandas versions? (with vbench there was
> the start date you could give with the benchmark)
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/pydata/pandas/pull/10998#issuecomment-139829603.
 In the "all benchmarks" view, there's a "Regressions" button in the upper
left. It's not spitting out anything due the tag issue I mentioned earlier.
On Sep 12, 2015 4:59 PM, "Jeff Reback" notifications@github.com wrote:

> @jorisvandenbossche https://github.com/jorisvandenbossche I put that
> one on the 0.17.0 RLS issue. How do you 'find' regressions though (e.g.
> except for looking at charts directly). Did I not do something?
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/pydata/pandas/pull/10998#issuecomment-139829697.
 ahh I c

ok I did it with the mark I eyeball :)
 @jreback In Christophers version it works, so you can see there how it looks: http://qwhelan.github.io/pandas_asv/#regressions?sort=3&dir=desc

But for checking a release, I think the `asv continuous` between the previous release and the current release is also an good option to have an overview of possible regressions
 @qwhelan also maybe I am using it wrongly

but asv gh-pages crashed for me

I just did: asv publish

the copied the html dir 
 No, the regression page not working is definitely a bug in master. Also, I
did not use asv gh-pages (didn't realize it was there). I did the pages
repo setup manually but I can investigate the crash tomorrow.
On Sep 12, 2015 5:20 PM, "Jeff Reback" notifications@github.com wrote:

> @qwhelan https://github.com/qwhelan also maybe I am using it wrongly
> 
> but asv gh-pages crashed for me
> 
> I just did: asv publish
> 
> the copied the html dir
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/pydata/pandas/pull/10998#issuecomment-139830954.
