feat: Solar wind and chromatic deterministic functions

[danielreardon]

Added solar wind GP code copied from enterprise_extensions methods.

Deterministic functions added for:

• Chromatic exponential event
• Chromatic Gaussian event
• Chromatic annual
• Orthometric Shapiro delay (full signal, so this is to be used when there is no Shapiro delay in the timing model)

Default priors added for all models

[meyers-academic]

@danielreardon Ok, I have added some things. I pushed it to the branch on your fork that you open the PR with, I hope that's ok(I think you clicked a button that let me do that). If not, then the next time I can open a branch on my own person fork and ask to merge to your branch separately.

I added some unit tests and doc strings and fixed a circular import that it ran into.

This is all a bit of an updated workflow for me, so I'm trying as we add things to make sure we have tests covered and docstrings...and I'll go back through all the old code eventually and hopefully update that as well.

[meyers-academic]

I think what is left to do is

• doc strings for solar.py updates
• tests for solar.py updates

I also pulled the sphinx documentation commits on the main branch so that we can check how the docstrings render.

[meyers-academic]

I think this is basically ready to go. I've pretty much just updated the doc strings and the tests. @danielreardon can you take a look? I added an example also for the fourierbasis_solar_dm, as understood how this is to be used. The tests/test_solar.py (which I used AI to help make and then looked through) all look sensible, including testing the time domain GP example (you might look at that one specifically to see if the docs agree with your vision, and if tests do as well).