Only add 2nd Best Fit option if it actually exists

[weaverba137]

This PR fixes #98.

When Prospect is used with SDSS data or data derived from SPARCL, a 2nd best fit model is not available. However, ViewerWidgets.add_model_select() was assuming that it was always available based on with_full_2ndfit which defaults to True.

This resulted in the JavaScript side of prospect attempting to obtain data from an object that is null (a Python None converted to JavaScript).

With this change, the argument with_full_2ndfit is unused, can it be eliminated from that method entirely?

[coveralls]

coverage: 2.008% (-0.002%) from 2.01%
when pulling 0c76f6e on missing-2nd-model
into aa69275 on main.

[armengau]

Agree with the change (and removed arg with_full_2ndfit).

Now I understand what happened: indeed you had redrock_cat=None, an option previously incompatible with with_full_2ndfit=True (which is default...). I never stubbled on that because the script prospect_pages.py explicitely prevented this, with some hard-coded settings in the script.

I took this opportunity to remove this hard-coding.

I tested the pages quite extensively, on my side it's ok now.

[armengau]

.. But please check one last time on your notebook!

[weaverba137]

Testing now. If all goes well, I'll merge shortly.

[weaverba137]

Tests look good, so I will merge momentarily.