Changes
Improvements in pandas DataFrame, DataFrameGroupBy and SeriesGroupBy
- specify DataFrame.groupby
- add DataFrameGroupBy.aggregate
- adjust data type in `DataFrame.__init__`
- add `__getattr__` to get columns in DataFrame and DataFrameGroupBy
- correct return type of `DataFrameGroupBy.__getitem__`
- add some missing statistical methods to DataFrameGroupBy and SeriesGroupBy