So that:
- developers can understand the logic and write new routines that
  confirm (or something else otherwise)
- Sphinx documentation displays the description; this helps users
  and developers understand the routines

Addresses postprocessor!10 (comment 94319)

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.

Former-commit-id: 2b17ee6a922e2103a398fa3f5e6e23de1075fb96

Former-commit-id: 184f18f71b1d0ce12f1ef25e4a3a9b040f3cd828

Gaussian Process currently fixed to growth rate, could be used for
any signal in theory so the names might be changed later.
Also edited the processor so it can deal with a process that returns
multiple signals.


Former-commit-id: cd1322c7e17c1e041d9cb99186ca26c8451935c5