Add plotting routines

Users can import plotting routines and use them on DataFrames for data
visualisation purposes, e.g.:

    from postprocessor.routines.<type> import <type>
    <type>(df, **args)

or:

    import postprocessor.routines as ppr
    ppr.<type>.<type>(df, **args)

I essentially copied over
https://git.ecdf.ed.ac.uk/swain-lab/aliby/skeletons/-/blob/arin/scripts/users/arin/alibyplot.py,
but divided each plotting type into its own file to make it more
organised.

Currently, it's fully functional for any users to use, but there are
issues:
- General issues:
    - Code repetition, e.g.
        - Defining attributes in __init__
        - Defining Axes
    - Need to call ppr.<type>.<type> -- would be more convenient if
      ppr.<type> as if the user is using matplotlib.pyplot or seaborn.
    - Defaults, e.g. plot titles, are very specific to what Arin does.
- Specific issues:
    - By default, the heatmap creates Axes with the left half filled
      with whitespace.  This doesn't affect visualisation, but it's a
      minor annoyance.

I plan to eliminate code repetition by applying OOP/design patterns
later.

Addresses issue #14.