How do we associate/discover PCDM:Files related w/ iana:describes?

[afred]

rationale: The current thinking is to associate one or more files containing metadata with the binary files they describe using iana:describes predicates between PCDM:File objects.

But there is still some unknowns (for me at least) as to what that looks like in Ruby, Fedora, and Solr.

Done when: we can demonstrate one or more approaches on how to associate a metadata file with the file it describes using iana:describes predicate.