Cleanup of model registry

[JostMigenda]

Two points that were left over from #184 but are important to fix before a final release:

• [ ] update Jupyter notebooks and “Getting Started” page of documentation with new way of initialising models
• [ ] Clean up the models in ccsn, base and loader. (E.g. unused _RegistryModel base class, duplicate code in loader classes)

As part of cleanup, see also this comment by @Sheshuk.