Implementing preSN registry models

[Sheshuk]

Closes #243
Closes #290

• Fix model tests which were not using absolute file paths (test_02_models.py)
• Moved GarchingArchiveModel from base to the loaders module.
• Created new LocalFileLoader class for checking if the requested file exists
• And RegistryFileLoader to download the requested file from the _model_downloader

Todo:

• Implement RegistryModels for preSN
• Add tests for preSN models
• Add preSN models to the documentation

[JostMigenda]

I’ve finished looking through the code-level changes; they all look good to me. Thanks a lot, @Sheshuk!

Before I accept and merge this PR, I’d like to try some usage examples. I noticed that the demo notebooks (e.g. https://github.com/SNEWS2/snewpy-models-presn/blob/master/models/Patton_2017/Patton.ipynb) still use the old (filename-based) initalisation, which doesn’t work with this PR. Have you started updating those already, or should I go ahead, update them as I try them out and make a PR on the snewpy-models-presn repo?

[Sheshuk]

Thanks @JostMigenda! I will update these examples, they're rather old in general.

[JostMigenda]

Nice work, thank you!