GSL warnings for discrete SCF functions

[TomWagg]

Describe your changes

I've added the same check that compute_coeffs has to compute_coeffs_discrete so that you get an error message rather than a list of zeros as your output if GSL_ENABLED is False.

I also added some warnings to the docstrings about the needs for GSL for these functions, obviously feel free to delete those.

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 for catching this! I'll merge when CI passes