Create docstring for DataFrameGroupBy.describe and reimplement leveraging _InternalFrame

[deepyaman]

Close #1166

[deepyaman]

Honestly not sure what's wrong here. :( I've tried many variations locally, including fixing the whitespace for a line myself, but it doesn't seem to work... (or I'm overlooking something stupid) Yep, something stupid. =/ Fixed.

=================================== FAILURES ===================================
________ [doctest] databricks.koalas.groupby.DataFrameGroupBy.describe _________
194            a  b  c
195         0  1  4  7
196         1  1  5  8
197         2  3  6  9
198 
199         Describing a ``DataFrame``. By default only numeric fields
200         are returned.
201 
202         >>> described = df.groupby('a').describe()
203         >>> described  # doctest: +NORMALIZE_WHITESPACE
Differences (unified diff with -expected +actual):
    @@ -1,5 +1,5 @@
    -      b                                        c
    -  count mean       std min 25% 50% 75% max count mean       std min 25% 50% 75% max
    -a
    -1   2.0  4.5  0.707107 4.0 4.0 4.0 5.0 5.0   2.0  7.5  0.707107 7.0 7.0 7.0 8.0 8.0
    -3   1.0  6.0       NaN 6.0 6.0 4.0 6.0 6.0   1.0  9.0       NaN 9.0 9.0 9.0 9.0 9.0
    +      b                                             c
    +  count mean       std  min  25%  50%  75%  max count mean       std  min  25%  50%  75%  max
    +a
    +1   2.0  4.5  0.707107  4.0  4.0  4.0  5.0  5.0   2.0  7.5  0.707107  7.0  7.0  7.0  8.0  8.0
    +3   1.0  6.0       NaN  6.0  6.0  6.0  6.0  6.0   1.0  9.0       NaN  9.0  9.0  9.0  9.0  9.0
/home/travis/build/databricks/koalas/databricks/koalas/groupby.py:203: DocTestFailure

[deepyaman]

@itholic @HyukjinKwon I believe this is ready. I wasn't able to use name_like_string as in #1168 (comment); the underlying columns are like ('b', 'count'), not (b, count). I think this is similar to how agg is implemented, anyway.

[codecov-io]

Codecov Report

Merging #1202 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1202   +/-   ##
=======================================
  Coverage   95.26%   95.26%           
=======================================
  Files          35       35           
  Lines        7154     7154           
=======================================
  Hits         6815     6815           
  Misses        339      339

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	91.9% <100%> (ø)	⬆️
databricks/koalas/usage_logging/__init__.py	97.29% <0%> (ø)	⬆️
databricks/koalas/usage_logging/usage_logger.py	100% <0%> (ø)	⬆️
databricks/koalas/__init__.py	85.1% <0%> (ø)	⬆️
databricks/conftest.py	98% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 72c0d04...36b77db. Read the comment docs.

[HyukjinKwon]

Release is being close. I will just merge. @itholic, do you have some time to add the test and double check the PR?

[itholic]

@HyukjinKwon okay i'll manage this until tomorrow