Adds a factory options to :pbcore_desc_doc factory to build pbcoreIdentifier nodes with a value for the 'source' attribute.
Refactors Openvault::Pbcore::AssetRelationshipBuilder#relate out into two methods.
Moves logic to relate assets together out of each individual model class, and into the AssetRelationshipBuilder class.
Adds descriptive error message for UnhandledRelationshipType errors.
Changes spec to not abuse subject.
Fixes some remnant bugs due to changes to the api of Openvault::Pbcore::AssetRelationshipBuilder
Merge pull request #977 from afred/refactor-ingest-spec

Adds a factory options to :pbcore_desc_doc factory to build pbcoreIdenti...