Add python testing/linting/formatting framework for buildnml?

This may not be true of all component buildnml's. But, at least the DATM buildnml has a ton of options and a lot of complexity with how the streams work and the namelists created. In CTSM we have a nice framework where python scripts can have unit and system tests, as well as linting and formating through black. It would be good to have something like this in place in CDEPS as well.

I'm starting this as more of a discussion than anything else. This is certainly a post ctsm4.0 release activity.

Start by reviewing the CTSM Python testing framework and the DATM buildnml scripts, especially their options, streams, and generated namelists. Define the scope for unit tests, system tests, linting, and Black formatting; done means an agreed framework is in place for the relevant CDEPS buildnml components.

Written by the indexing model from the issue text.