FEATURE: Plotting routines

Summary

Add plotting routines to postprocessor.routines.

Current behaviour/setbacks

We don't have integrated plotting routines and rely on individual users' scripts, which can be all over the place.

Desired behaviour/advantages

postprocessor.routines has plotting routines.

Advantages:

• We will be creating a place where users can share and add their plotting routines -- saves re-inventing the wheel.
• Users can easily invoke them when analysing segmented & post-processed data, for example, in a user script in skeletons. This will speed up analysis.

Implementation sketch

Copy routines from https://git.ecdf.ed.ac.uk/swain-lab/aliby/skeletons/-/blob/arin/scripts/users/arin/alibyplot.py with each type of plot going to a separate file.