Feature/pytest unit tests clean

[massimolarsen]

Summary of changes

This PR updates the unit test suite to run under pytest directly, replacing the old test/unit/run.py entry point with pytest collection and an option for python and numba execution. It renames existing unit tests to pytest discoverable test_*.py files, adds shared fixtures for distribution, geometry surface, and tally tests, and updates the GitHub unit-test workflow to run python -m pytest test/unit. Some existing tests (mainly the distribution tests) are updated to run properly in numba mode.

pytest test/unit now automatically runs unit tests in both pure python and numba mode.
pytest test/unit --mode=python runs tests in pure python mode.
pytest test/unit --mode=numba runs tests in numba mode.

Localized it to just 'pytest test/unit' at this point in time. I will be updating regression tests soon, so I want to keep this hook separate for now. This is fine with Github because the workflows runs 'pytest test/unit'. Could be changed in the future though.

Types of changes

• Organization and beautification (changes which improve readability and/or accessibility)

Developer Checklist

• I have read the contributing guide.
• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests pass

Associated Issues and PRs

• closes #
• related to #

Associated Developers

• Dev: @massimolarsen