Should find a fix to avoid needing mpi4py + openmpi to run tests

[davewhipp]

Currently the mpi4py and openmpi packages have been included in the [tests] optional dependency in the pyproject.toml file because the automated tests fail on GitHub Actions without them. This seems to be due to init_params() and prep_model() being imported in the __init__.py file in the tc1d package directory.

I don't think these should be needed for testing, as no tested code (so far) depends on MPI. Thus we should either:

1. Find a way to avoid importing init_params() and prep_model() in the __init__.py file, or
2. Modify the testing to somehow only import them when not running the test suite