Work on Catalog Data Structure

[jeafreezy]

Work on the data structure for registering a catalog e.g via YAML, JSON, CSV.

[jeafreezy]

YAML support has been added and the structure has been defined in .superstac.yml. It also support users passing their own config file provided is conforms with the superstac data structure.

CatalogManager.register_catalog will be exposed to allow easy registering programatically too.

At this point, I don't think a CSV support is necessary.

The data structure is in a good state for now, just needs some review and documentation update.