Added ddof parameter for GroupBy.std() and GroupBy.var()

[itholic]

Added missing parameter ddof for GroupBy.std() and GroupBy.var().

>>> kdf = ks.DataFrame(
...     {
...         "a": [1, 2, 6, 4, 4, 6, 4, 3, 7],
...         "b": [4, 2, 7, 3, 3, 1, 1, 1, 2],
...         "c": [4, 2, 7, 3, None, 1, 1, 1, 2],
...         "d": list("abcdefght"),
...     },
...     index=[0, 1, 3, 5, 6, 8, 9, 9, 9],
... )
>>> kdf
   a  b    c  d
0  1  4  4.0  a
1  2  2  2.0  b
3  6  7  7.0  c
5  4  3  3.0  d
6  4  3  NaN  e
8  6  1  1.0  f
9  4  1  1.0  g
9  3  1  1.0  h
9  7  2  2.0  t

# std
>>> kdf.groupby("a").std(ddof=1)
          b         c
a
7       NaN       NaN
6  4.242641  4.242641
1       NaN       NaN
3       NaN       NaN
2       NaN       NaN
4  1.154701  1.414214

>>> kdf.groupby("a").std(ddof=0)
          b    c
a
7  0.000000  0.0
6  3.000000  3.0
1  0.000000  0.0
3  0.000000  0.0
2  0.000000  0.0
4  0.942809  1.0

# var
>>> kdf.groupby("a").var(ddof=1)
           b     c
a
7        NaN   NaN
6  18.000000  18.0
1        NaN   NaN
3        NaN   NaN
2        NaN   NaN
4   1.333333   2.0

>>> kdf.groupby("a").var(ddof=0)
          b    c
a
7  0.000000  0.0
6  9.000000  9.0
1  0.000000  0.0
3  0.000000  0.0
2  0.000000  0.0
4  0.888889  1.0

[xinrong-meng]

Looks great! Thanks!

[ueshin]

What about GroupBy.var()? Shall we do the same thing for it here?

[itholic]

Sounds good. done !

[ueshin]

LGTM, pending tests.

[codecov-io]

Codecov Report

Merging #1994 (9fbec86) into master (ae5c8d8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1994      +/-   ##
==========================================
- Coverage   94.52%   94.51%   -0.01%     
==========================================
  Files          50       50              
  Lines       10952    10965      +13     
==========================================
+ Hits        10352    10364      +12     
- Misses        600      601       +1     

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	91.60% <100.00%> (+0.02%)	⬆️
databricks/koalas/generic.py	92.60% <0.00%> (-0.01%)	⬇️
databricks/koalas/missing/frame.py	100.00% <0.00%> (ø)
databricks/koalas/missing/series.py	100.00% <0.00%> (ø)

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[itholic]

Thanks! merging :)