Implement Pytest-cov, and make coverage more meaningful

[MicahGale]

This is related to #532. This should:

1. Move from coverage to pytest-cov for better feature support.
2. Restrict coverage reports to unit testing based suites, and excluding integration testing.

[MicahGale]

For restricting the coverage results I think we can:

1. organize tests into tests/unit and tests/integration folders. This does still make pytest happy.
2. Run something like coverage run -m pytest tests/unit followed by pytest tests/integration

[MicahGale]

I think the best way to do this is to map a unit test file to a source path:

{"test_cell_problem.py" : `montepy/cell.py` ...
}

Then we run pytest --cov for everything like:

pytest --cov=montepy/cell.py --cov-append tests/unit/test_cell_problem.py

This way code that is ran to setup fixtures will most likely not be erroneously counted.

While at it, branch coverage should be enabled.

Overall this won't be easy to implement effectively and will require a moderate amount of effort to:

1. Create the mappings
2. Switch to purely pytest-onic test suites.
3. Refactor the tests to better have unit tests be consolidated (I'm looking at you test_integration.py)
4. Write more exhaustive (property based included Implement Property based Testing #534) unit tests
5. parametrize all tests
6. extend parametrized tests to be property based tests.
7. Reexamine edge cases and if they are better suited as unit tests or other testing.
8. Change paradigm that new bugs are usually a failure of unit testing, and more integration tests are likely not the solution.

[MicahGale]

For restricting the coverage results I think we can:

1. organize tests into tests/unit and tests/integration folders. This does still make pytest happy.
2. Run something like coverage run -m pytest tests/unit followed by pytest tests/integration

In testing with #536, this dropped coverage to 89%