tests + automated testing with CI

[ShantanuKodgirwar]

@dboonz Since our group is actively using PtyLab here, I am keeping it a bit more stable (no planned major refactoring though). This moves us to v0.2.3. Here is the list of things this PR does:

• Moved all test files scattered across PtyLab/ subdirectories into a top-level tests/ directory (standard practice)
• Converted tests to pytest style to unify testing
• Added tests optional dependency group (pytest, imageio) to pyproject.toml; updated CONTRIBUTING.md. Further makes the dependencies leaner
• Added .github/workflows/test.yml running pytest on Python 3.12 and 3.13 for automated testing at every PR and addresses Add Github actions for testing #57

Minor refactoring that doesn't affect API or existing implementations:

• Perhaps an older implementation uses params.propagator (which I observed in a lot of use cases of people here), however this does nothing and defaults to Fraunhofer. I simply created this as an alias of params.propagatorType, so nothing changes and now both work the same.
• other minor bugs