Model Fitting Cleanup

[duytnguyendtn]

With the introduction of #456, we now have a good foundation for some cleanup of the model fitting plugin. This PR implements the following changes:

1. The model can be automatically added to the spectral viewers immediately without fear of this looping bug that's been plaguing us
2. The "Apply to Cube" option only applies to the "Cubeviz" configuration, and therefore is only visible in Cubeviz
3. With the introduction of Implement extraction of model fitting parameters #458, we can now extract models via code. This, combined with MAST's request that we don't allow saving to local resources in anticipation of Cloud Hosting, means the "Save to Disk" button is removed

I reviewed the docs, and these changes aren't actually mentioned in the docs, so there's nothing to change as far as I can see

[rosteen]

Looks good, thanks!

[camipacifici]

Looks great!