Add class bf:Danacode as subclass of bf:Identifier

[NancyL]

Danacode is a unique numeric commercial book identifier used mainly in Israel. It is recognized and registered as Standard Identifier Source Code at https://id.loc.gov/vocabulary/identifiers/danacode.html. Our catalogers find these identifiers regularly in Israeli books and would like to add them as identifiers.

The general website for Danacode is at: www.danacode.com

[klngwll]

Just for reference, In Libris we also added Danacode as a subClass of Identifier in our KB Vocabulary at the request of catalogers. So +1 on usage.

https://id.kb.se/vocab/Danacode

[kirkhess]

If we're adding identifiers, I think it is time to add OCN as a subclass of bf:Identifier, aka: https://id.kb.se/vocab/WorldCatNumber.

See lcnetdev/bibframe2marc#13 - this would assist in fixing this problem.

[jodiw01]

This was addressed in #44

"All members of the Identifier Sources scheme are now of type bf:Identifier to make clear their usability in Bibframe."