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.. -- would be more convenient if ppr. 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 (closed).