Adding placeholder GitHub CI unit tests to this repo

[john-science]

This is just a starting point, so we can hook unit tests into the Continuous Integration tools provided by GitHub.

I also made the (arbitrary) choice to add a tox.ini to make running CI easier to do the same locally as on CI. This is mandatory, if you hate it.

Notice I only added one silly unit test. Eventually, we can flush out this system with more CI tools:

• Adding more unit tests
  • The biggest hurdle I found here was many files did import openmc, which we will have to build into the CI workflow.
• Adding the ruff linter to the CI
  • Which I can open another CI for, that will look like this.

[john-science]

You'll also notice that now the "checks" for this CI show up on the bottom of every PR.