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
(cherry picked from commit d7b69946cb)
Signed-off-by: Reynold Xin <rxin@databricks.com>
