ENH: Implement MultiIndex.is_monotonic_decreasing
- [X] closes #16554
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Note that this is a dupe PR of #16573, which appears to have gone stale.

Implementation ended up being easier than what I suggested in the issue; can use the fact that an index is monotonic decreasing if it's reverse is monotonic increasing.  Just a matter of reversing the index and calling `is_monotonic_increasing`.

Regarding `MultiIndex` tests:
- Added a test case to the `is_monotonic_increasing`  test to verify it's working for an empty index.
- The `is_monotonic_decreasing` test cases consist of decreasing versions of the test cases in the `is_monotonic_increasing` test.

Regarding `IntervalIndex` tests:
- Added a test for `IntervalIndex.is_monotonic_increasing` and `IntervalIndex.is_monotonic_decreasing`  since they use the MultiIndex implementation under the hood.
- Split a test that originally checked both monotonic and `is_unique` into separate tests.  Expanded the test cases for both tests.
lgtm. @TomAugspurger can you give a once over. This looks fine, though I wonder if we can mirror the implementation of `is_monotonic_increasing` and avoid the overhead of reversing the index?

```python
    @property
    def is_monotonic_decreasing(self):
        """
        return if the index is monotonic decreasing (only equal or
        decreasing) values.
        """
        values = [self._get_level_values(i).values
                  for i in reversed(range(len(self.levels)))]
        try:
            sort_order = np.lexsort(values)
            return Index(sort_order).is_monotonic_decreasing
        except TypeError:

            # we have mixed types and np.lexsort is not happy
            return Index(self.values).is_monotonic_decreasing
```

@jschendel does that pass the tests you wrote, or am I missing something? maybe make a helper function for this? (and just call in both)

e.g. 

```
    def _is_monotonic_attribute(self, attr):
        values = [self._get_level_values(i).values
                  for i in reversed(range(len(self.levels)))]
        try:
            sort_order = np.lexsort(values)
            return getattr(Index(sort_order), attr)
        except TypeError:

            # we have mixed types and np.lexsort is not happy
            return getattr(Index(self.values), attr)
``` Yeah, it's borderline too much duplication to implement them both like that. I could go either way, but would probably have a helper. @TomAugspurger :  That doesn't quite work if the MultiIndex has dupes, e.g. `pd.MultiIndex.from_tuples([(3, 3), (2, 2), (2, 2)])`.  The issue is that `np.lexsort`  keeps the the dupes in order, so `sort_order` ends up being `[1 2 0]` in the case of my example.

The workaround I came up with was to add a fake unique decreasing level to `values` to force uniqueness, then use the approach you suggested.  I actually implemented this when I was working on a solution before I realized I could reverse.

I did some ad hoc timings to decide which implementation to use, and the overhead of reversing the index was actually less than the overhead of adding an additional level to sort by in `np.lexsort` once the MI got sufficiently large (fairly significant when I tested it on an MI with ~12 million elements).  Reversing was slower for smaller MI, but the gap wasn't terribly large.  Didn't do extensive timing though.

For reference, here's my implementation with the fake decreasing level to force uniqueness.  Perhaps there are some optimizations I missed. 

```python
def is_monotonic_decreasing(self):
    """
    return if the index is monotonic decreasing (only equal or
    decreasing) values.
    """

    # initialize values as decreasing range to force uniqueness, needed
    # since lexsort() only sorts ascending and want dupes in decreasing order
    values = [np.arange(len(self) - 1, -1, -1)]

    # reversed() because lexsort() wants the most significant key last.
    values += [self._get_level_values(i).values
               for i in reversed(range(len(self.levels)))]
    try:
        sort_order = np.lexsort(values)
        return Index(sort_order).is_monotonic_decreasing
    except TypeError:

        # we have mixed types and np.lexsort is not happy
        return Index(self.values).is_monotonic_decreasing
```

I guess we could potentially use some type of heuristic based on size to choose between which method to use (similar to how `isin` is implemented iirc?) but that seems like it might just be unnecessary complexity. @jschendel can you post an asv of the is_monotonic for various index types (maybe against 0.20.3) @jreback : I haven't used asv before but it doesn't look too hard to use, so I should be able to get some benchmarks.

I'm a little confused as to what you're looking for?  I'm not changing the `is_monotonic` or `is_monotonic_increasing` methods (dupes are handled fine in the increasing case, it's just decreasing where they cause an issue).  Am I missing something?

Are you looking for an asv of `is_monotonic` and the two `is_monotonic_decreasing` methods I described (reversing and adding a fake level for uniqueness)?  Then compare the two `is_monotonic_decreasing` methods against each other to see which is better, and both against `is_monotonic` to see how much of a perf hit is taken by having to account for decreasing? @jschendel 

I think we already have *some* asv's for is_monotonic and friends, just want to be sure we have coverage for MultiIndex.  @jschendel I think this was fine, can you rebase @jreback : rebased

Regarding the asv's for `is_monotonic`:  there is one for `MultiIndex.is_monotonic`.  Looks like it's checking the non-mixed case (i.e. the `try` block).  Couldn't find any other asv's for monotonic at all, regardless of index type.  I might not be looking in the right place though? that looks like the place; I think we should have asv for these properties. it can do later if u want

this PR.lgtm ping on green # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=h1) Report
> Merging [#17455](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6630c4eddf2762d519507304ad73de189a7e0c6c?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17455/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17455      +/-   ##
==========================================
- Coverage   91.22%    91.2%   -0.02%     
==========================================
  Files         163      163              
  Lines       49625    49625              
==========================================
- Hits        45270    45261       -9     
- Misses       4355     4364       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <100%> (ø)` | :arrow_up: |
| #single | `40.19% <50%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.9% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=footer). Last update [6630c4e...ae6320b](https://codecov.io/gh/pandas-dev/pandas/pull/17455?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ping @jreback : green Thanks @jschendel!