106 outlying photometry checker

[jrob93]

Fixes #106

I have added some simple and generic utility functions to detect outliers (e.g. outlying photometric wrt a phase curve model). I need to refine the example notebook and add unit tests.

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?

[astronomerritt]

This looks good to me, but I'd also like Meg to look at it.