Allow direct assignment in DataFrame.index

[HyukjinKwon]

Pandas allows assignment to DataFrame.index

>>> import pandas as pd
>>> df = pd.DataFrame({'a': [1, 2, 3]})
>>> df.index = ['a', 'b', 'c']
>>> df
   a
a  1
b  2
c  3

whereas Koalas seems unable.

>>> import databricks.koalas as ks
>>> df = ks.DataFrame({'a': [1, 2, 3]})
>>> df.index = ['a', 'b', 'c']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

[HyukjinKwon]

This seems to be difficult because the operations between different DataFrames are dependent on index.

df.index and ['a', 'b', 'c'] are different datasets so we should figure out a way to join both by a certain axis column.