phase curve plot func

[jrob93]

Fixes #112.

This is a start at adding some plotting utility functions, e.g. making phase curve plots from adler objects. I have tried to write this such that a matplotlib figure object is returned by the function, this can either be displayed or manipulated further.

Review Checklist for Source Code Changes

• [ ] Does pip install still work?
• [ ] Have you written a unit test for any new functions?
• [ ] Do all the units tests run successfully?
• [ ] Does adler run successfully on a test set of input files/databases?
• [ ] Have you used black on the files you have updated to confirm python programming style guide enforcement?

[jrob93]

If this approach looks sensible I would like to add more plotting functions and improve the demo notebook notebooks/plotting_utilities.ipynb (and add unit tests of course)