Allow more levels in the ModelRegistry YAML configuration tree

[Sheshuk]

What we have now

All models are listed in models section of the model_files.yml: https://github.com/SNEWS2/snewpy/blob/e0e47f500185fe71c801a6495d9526cad74e830c/python/snewpy/models/model_files.yml#L8-L14

That's ok for the ccsn case, but for adding the preSN models I would like to have them in a separate list.

What I suggest

Make a tree like

models: 
    ccsn:  
        Bollig_2016: 
            repository: *snewpy 
        Fornax_2019:  
            repository: *snewpy 
    presn:
        ...

and make the _model_downloader search not only by the model name, but also its module (ccsn or presn).

This will keep more order than just having all the models in one list.

[sybenzvi]

This sounds fine if we feel it's useful to track the different emission models.

If we're going this route, would it be useful to rename the new repository snewpy-models-ccsn to snewpy-models and create ccsn and presn top-level folders in that repo?

[JostMigenda]

Note that we already have a (currently private) repo called snewpy-models-presn. I have no strong opinions on whether we want to merge those model files into the new repo or keep the two repos separate; but if we're worried about the file size in the longer term as we add more and more models, keeping them separate would help a bit with that.