dataframe.py is splited into column.py, group.py and dataframe.py:
```
   360 column.py
  1223 dataframe.py
   183 group.py
```

Author: Davies Liu <davies@databricks.com>

Closes #6201 from davies/split_df and squashes the following commits:

fc8f5ab [Davies Liu] split dataframe.py into multiple files