Support SeriesGroupBy.filter.

[ueshin]

Adding SeriesGroupBy.filter.

>>> df = ks.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar', 'foo', 'bar'], 'B' : [1, 2, 3, 4, 5, 6], 'C' : [2.0, 5., 8., 1., 2., 9.]})
>>> df.B.groupby(df.A).filter(lambda x: x.mean() > 3.)
1    2
3    4
5    6
Name: B, dtype: int64

[ueshin]

This is based on #1480. Please review it first.

[codecov-io]

Codecov Report

Merging #1483 into master will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1483      +/-   ##
==========================================
- Coverage   93.82%   93.81%   -0.01%     
==========================================
  Files          36       36              
  Lines        8433     8442       +9     
==========================================
+ Hits         7912     7920       +8     
- Misses        521      522       +1     

Impacted Files	Coverage Δ
databricks/koalas/missing/groupby.py	100.00% <ø> (ø)
databricks/koalas/groupby.py	89.40% <83.33%> (+0.01%)	⬆️

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[itholic]

Looks good to me If #1480 is approved.

[ueshin]

Thanks! I'd merge this now. Please feel free to leave a comment if any.