Add a runtime type checker

[rg936672]

What's the issue?

A checker such as beartype would provide an extra layer of assurance that our type hints are actually correct. Runtime type checkers will be able to examine more complex type hints (such as jax array shapes as added in #746), which is an advantage over Pyright (#427). However, there currently doesn't seem to be any reason we can't have both! If performance is a concern, we can only run the runtime checker while running the test suite, but beartype is apparently fairly speedy so we could include it in the live package as well.

To check: beartype is already listed as a test dependency. Is this really a test dependency, or should it be moved to dev? Or change dependency to pytest-beartype?

[tp832944]

Probably not worth doing until after #427, but then make it fairly high priority.