Implement Chen+24 particle spray model

[ybillchen]

Describe your changes

This branch implements the new Chen+24 (arXiv:2408.01496) model for stream modeling. A notebook demonstrating how to use the model is available.

-Bill Chen

Checklist

• Did you add tests?
• Did you add documentation for your changes?
• Did you reference any relevant issues?
• Did you add a changelog entry? (see CHANGES.rst)
• Are the CI tests passing?
• Is the milestone set?

[adrn]

Thanks @ybillchen -- this is looking great! I fixed some issues related to numpy 2.0, so to get the tests to pass you will need to rebase this PR. Let me know if you want help doing that!

[adrn]

I went ahead and did the rebase for you!

[adrn]

Thanks @ybillchen!

[ybillchen]

Thank you Adrian! Super happy that this is merged to gala!