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)

[astronomerritt]

This looks good to me. Looks like it'll be fairly simple to add more plotting functions. Seems very flexible too.

Regarding your comment on Slack: this does need to be unit-tested! I will approve without the unit test but here is how I would do it:

• Run the function several times testing each if-statement (so one that uses label_list, one that uses col_list, one accepting fig as an argument). You don't have to check the resultant plots - the aim of the unit test is to make sure that no errors trigger when the function is called. That way, if anything changes in the code that would break plotting entirely, we'll know.
• Create a plot using savefig then check that it exists where you told it to save using os.path.exists(). Using pytest's tmp_path functionality is helpful here to direct output of tests to a temporary location. You can see how that's done in test_AdlerData.py in the test test_write_row_to_database().