Provide API for fetching linked catalogs

[delucchi-cmu]

Current mechanism for using the almanac to find e.g. a linked source catalog is bulky. If this is a pattern we expect to support, we should write a simpler interface.

Instead of:

    source_almanac = default_almanac.get_almanac_info(object_almanac.sources[0].catalog_name)

possibly:

    source_almanac = object_almanac.get_sources()

[maxwest-uw]

self-assigning this as an excuse to get ready for margin cache changes 😎