Model registry cleanup

[JostMigenda]

This fixes #224. Two things I want to highlight in particular:

• It implements this suggestion by @Sheshuk to move the parameter checking into the appropriate base class
• It removes the Model.param_combinations property in favour of Model.get_param_combinations() (which already existed anyway). In addition to saving some boilerplate code, this is also more extensible. For example, I could imagine adding the ability to give additional arguments like get_param_combinations(eos='shen') to narrow the list down.

During the review, please note that thus far, I’ve only updated Nakazato_2013.ipynb with the new model initialisation. Let me know if that looks reasonable; if so, I will update

• [x] the notebooks for all other models and
• [x] the Getting Started page of the documentation

before we can merge this PR.

[github-actions[bot]]

Result of Benchmark Tests

Benchmark	Min	Max	Mean
python/snewpy/test/test_05_snowglobes.py::test_simulation_chain_benchmark	4.32	4.45	4.38 +- 0.05

[JostMigenda]

snewpy.models._init_model should be public-facing.

Eventually, yes. But for now, I think it’s not yet ready—see #225. For this PR, I just wanted to move that out of the util.py file so I could delete that as part of the cleanup.