Make ExaConstit a callable library

[rcarson3]

While working with a number of workflow tools, it is starting to seem like it might be useful to make ExaConstit into a callable library. I don't imagine this requiring too many changes. The big thing would be that we would want to take in the same arguments we do now as well as an MPI_Comm argument.

Long term this action might also allow us to drive things from Python and feed data back there as well. However, we would need to break things down quite a bit more to get to that state and do some major refactoring.

Just to put some context into things, we're currently exploring https://emews.github.io/ as a ways to parameterize models on machines like Summit and Frontier.