Create a set of helper functions for running customized functional tests/heuristics

In [FATES](https://github.com/NGEET/fates/blob/main/testing/run_fates_tests.py), we have a script that can run small programs (that you write) exercising pieces of the FATES code. This has been helpful for prototyping as well as verification/validation work. As discussed in a recent CSEG meeting, this script would be useful to other models.

I can work on modifying the FATES version so that it is model agnostic. Most likely this will be a class or set of methods you can call from your own model-specific script.