add Boccioli models

[JostMigenda]

During the SNEWS Collaboration Meeting, Luca Boccioli offered that we could add the simulations presented in his talk to SNEWPY.
I have a sample file and can write the necessary class; but with major changes to the SupernovaModel subclasses coming in #184, I’ll wait until that’s merged before completing this work & submitting the PR.

• write registry class (in models.ccsn)
• write loader class (in models.loaders)
• add Zenodo ID to model_files.yml
• add tests
• add documentation and example notebook
• (add support for this model to sntools)

[mcolomermolla]

I will also have two new models that I will add (bubli2021, 3D MHD with different magnetic fields, and Mastrototaro2019 with "heavy" sterile neutrinos). I am in contact with both authors which were happy to share the data. I already have the data of the first and will get the second one soon. Let me know if I can help speed implementing what is needed before this.