Add unit testing

[rsdefever]

We now have integrated the test suite into our development pipeline, which is a great first step. These are effectively integration tests. As the code continues to become more complicated we should also add unit tests. This will allow us to test the behavior of specific functions and increase our confidence that new code changes do not break existing behavior.

I need to do more research, but one option is pFUnit. An example of another scientific code that uses pFUnit is here. Look at the .pf files to get an idea of how it is implemented.

Please reply with other options below!