disable 'str' for 'SeriesGroupBy', disable 'DataFrame' for 'GroupBy'

[itholic]

as commented #1050 (comment) ,

we're gonna block generating SeriesGroupby by its column name to follow the behavior of pandas.

>>> import pandas as pd
>>> pser = pd.Series([1, 2, 3, 4, 5], name='x')
>>> pser.groupby(pser.name).head(2)
Traceback (most recent call last):
...
KeyError: 'x'

i think it's better to fix this in separated PR with a separated Issue to make it more clarify and add related tests for existing functions.

+

and also by should not allowed as type of DataFrame like below.

>>> pdf = pd.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])
>>> pdf.groupby(pdf)
Traceback (most recent call last):
...
ValueError: Grouper for '<class 'pandas.core.frame.DataFrame'>' not 1-dimensional

>>> pdf.a.groupby(pdf)
Traceback (most recent call last):
...
ValueError: Grouper for '<class 'pandas.core.frame.DataFrame'>' not 1-dimensional

[itholic]

I'm working on this.