Make JAX an optional dependency

[Lnaden]

This PR makes JAX an optional dependency at the pip level and raises a warning if JAX could not be imported to tell users it could be faster.

On the pip side, there will be 2 different instal options:

• pip install pymbar will install without JAX
• pip install pymbar[jax] will install with JAX.

Partial progress on #500. Until the conda feedstock issue is resolved with a new recipe, I want to leave #500 open to keep documenting it.

~Note: there is an outstanding TODO on the README.md file to fill in once we have an official name for the lite version on conda-forge.~ Resolved as pymbar-core

[codecov[bot]]

Codecov Report

Merging #503 (dcc6b6d) into master (2740798) will decrease coverage by 0.15%. The diff coverage is 86.66%.

[Lnaden]

The docs failure has to do with Sphinx-RTD-Theme not being compatible with Sphinx 7.0+, not something we can directly control unless we add a requirements.txt and pin sphinx for RTD. But then we have an requirements file which will kind of confuse the installer, so I dont want to do that. Some talk of changing to Furo, but different issue than here.