init_model convenience function

[JostMigenda]

The init_model convenience function was added in #184 to try out; but is currently hidden in snewpy.models.util, which is (a) not included in the docs and (b) probably not a submodule that we’d want to expose to the user.

There’s also Andrey’s comment about whether to require e.g. "ccsn.Nakazato_2013" as an argument instead of just "Nakazato_2013".

[JostMigenda]

I’ve moved the function to snewpy.models in #230.

[JostMigenda]

Also: Should this still use the get_models downloader (which downloads all progenitors for that model family) or should it use the new downloader (which only loads the requested progenitor)?

[JostMigenda]

Almost two years later, we’ve never actually needed this convenience function; so let’s remove this dead code.

[sybenzvi]

Remove the stub code (@JostMigenda ) and then close the ticket.