Add Continuous Integration to Public Branch

[DanWaxman]

This PR adds continuous integration to the public branch, and fixes linting errors. This is mostly derived as a git cherry-picking of #3. The main difference from #3 is that this applies changes directly to public, without any regard for the under-development numpyro API (or related DPF features).

• Add continuous integration, including linting and testing
  • Note: I didn't want to undertake significant refactors in dynamax, so these are ignored.
• Makes many small edits for linting
  • Warning: these are mostly LLM-generated refactors, as the workload would be enormous otherwise, but almost always for very simple and explicitly specified issues (e.g., "Resolve the linting errors outputted in lint_output.txt, including: [E722] use the correct exception (usually an AssertionError), [E402] move imports to the top where needed, [F821] add imports where missing").
  • I've checked all by hand, but please do double-check these.

Remaining to-do:

• Refactor tests to use pytest, be manageable on a CI machine, and make sure those pass.
• Consider doing type checking with mypy. This might be very high-effort though.

In general, one may now use make clean to run relevant ruff format and ruff check --fix commands (excluding notebooks and dynamax), make lint to lint, and make test to test.