add unit tests (and docstrings) to phase retrieval code

[douglase]

reading:

https://packaging-guide.openastronomy.org/en/latest/index.html https://carpentries-incubator.github.io/python-testing/ https://docs.astropy.org/en/latest/development/testguide.html#writing-tests https://numpy.org/doc/1.21/docs/howto_document.html