Add mode to dataframe and series

[V1NAY8]

Closes #215

• Added mode to dataframe and series

• Added tests, documentation

• Currently mode isn't supported by pandas groupby too, Hence added a NotImplementedError

• To satisfy mypy I had to add type hints to some other methods too.

• Implemented es_size parameter instead of dumping the entire cluster if we have multiple mode values.

• Currently dropna=True is only supported because I have a query on missing parameter of terms aggregation
  Reference

@sethmlarson Please review 😃

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[sethmlarson]

This is really great, here are some comments for you!

[V1NAY8]

@sethmlarson If you're happy with the change, I will rebase my branch and resolve merge conflicts :)

[sethmlarson]

jenkins test this please

[sethmlarson]

Sorry for letting this go so long without a final review, I'm pretty sure this is ready but want to run it against the new pandas v1.2.0, can you rebase?

[V1NAY8]

I forgot to include recursive-include eland in manifest.in. So, I included it. Otherwise the files won't come to release packages.

[V1NAY8]

@sethmlarson Ask jenkins to test this.

[sethmlarson]

jenkins test this please

[V1NAY8]

CI is good 🔥

[sethmlarson]

This is almost done! Here's a couple more comments:

[V1NAY8]

I had to make one small change to tests because
Even though the values are same, dtypes are being differed. I am hoping this shouldn't be a thing.
But dtype checks for other es_size are working.

>>> pd_testing.mode()[:1] 
  Cancelled  dayOfWeek           timestamp DestCountry FlightNum
0     False          0 2018-01-09 07:54:19          IT   00882F6
>>> ed_testing.mode(es_size=1) 
   Cancelled  dayOfWeek           timestamp DestCountry FlightNum
0      False          0 2018-01-09 07:54:19          IT   00882F6

assert_frame_equal(ed_testing.mode(es_size=1),pd_testing.mode()[:1])
AssertionError: Attributes of DataFrame.iloc[:, 0] (column name="Cancelled") are different

[left]:  bool
[right]: object

[sethmlarson]

Just one more comment, otherwise looks good to me!

[sethmlarson]

jenkins test this please

[V1NAY8]

Added those files.

[sethmlarson]

LGTM, thank you!