Splitting plots module into plotting subpackage

As title indicates, splits the huge plots.py file into the plotting subpackage with the following structure:

plotting/
    ```__init__.py``` - 79 lines
    ```dataplots.py``` - 826 lines
    ```pandas.py``` - 188 lines
    ```sheetplots.py``` - 284 lines
    ```viewplots.py``` - 945 lines

Now ready to be merged.