Adding UO Names Controlled Vocabularies
Adding new MARC Relator fields to data model
adding schema.org fields
Adding Holdings ontology and the holding:label term

holding:label is a call number or item label; mapped to GenericAsset#itemLocator.
Adding predicates from UO

...and removing percent for art names, which are redundant.
Additional metadata tweaks

...to bring model in line with upstream definitions.
Merge pull request #505 from OregonDigital/feature/metadata-updates

Feature/metadata updates