IO for SCF potentials

[TomWagg]

Describe your changes

I tweaked the potential __init__.py so that it can find SCF potentials, which ensures that IO will work for them. I also added a test for this (hopefully in the right place/correctly) which passes for me now.

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?

[TomWagg]

@adrn unsure what to do with the failing test - it fails because GSL isn't installed (makes sense) but not sure how to tell it to skip that test - would it make sense to just check gala._cconfig.GSL_ENABLED and only do the rest of the test if it is? Or is there something smarter?

[adrn]

In the test file, you can do from gala._cconfig import GSL_ENABLED and then skip the test if that is false. Examples here: https://github.com/adrn/gala/blob/main/gala/potential/potential/tests/test_all_builtin.py#L222

[TomWagg]

Cool! I knew you'd have a fancy method 🙃

It should be all set now!

[adrn]

Thanks @TomWagg !!