Prototype for a model-registration parser

[danielhuppmann]

A discussion for a model-registration reminded me of this prototype for a model-registration parser utility, so this PR adds the initial implementation for future use and improvement.

My suggestion is to to a quick review, then merge, and extend features, etc. as the need arises.

[phackstock]

I think there's a few updates necessary to make this functional and nice.
If you want you can go ahead with the merge as is but I'd suggest that I'll update your branch for clean up.

[phackstock]

Finally got around to finishing this one.

[phackstock]

I modified RegionAggregationMapping.from_file so that it chooses which parser to use depending on whether we read yaml or excel. In addition I added a to_yaml method so that you can write the class to a file.
With those two components parse_model_registration is nice and compact :)
@danielhuppmann, since this is your PR I think I can't chose you as a reviewer, is this ok or should I open it again under my name so that you can review it properly?

[danielhuppmann]

Thanks for continuing to work on my initial prototype, looks very nice!