Put model files in a separate repository

[Sheshuk]

Right now all the model data files are bundled in this repository, despite the new model registry functionality, which allows us to put them elsewhere.

As I suggested in #238 we can move out all the model files to a separate repository snewpy_models, and manage them there.

Advantages:

1. Cut new release of the snewpy_models when the data is changed, without having to wait for snewpy release.
2. snewpy repository will become very lightweight (no need to download all the models during the checkout, for example if you want to work on a specific feature, implement an interface to a new model etc.)
3. We can create Zenodo entries for the snewpy_models, and direct the downloader there, if needed.

I think it can be done quite fast and simple.
I implemented #247 to avoid hardcoding the repository path, and use any provided URL, so it should be easy to point to the new snewpy_models repo.

[JostMigenda]

I’ve created https://github.com/SNEWS2/snewpy-models-ccsn/ and added a model file that I’m going to use to implement #160. I’ll see what the workflow is like with this separate repo and think about how to automate some steps, especially around version tagging & snewpy releases.