interim solution for identifying the subject of metadata as an IRI

[graybeal]

With respect to #1208, we need an interim solution to capture the metadata subject IRIs in a way that can be post-processed to create the desired RDF or nanopubs.

Barbara Magagna suggests:

But I would advise to follow the FDO approach and Luiz has created for this an [ontology ](https://w3id.org/fdof/ontology) from which we could reuse the object properties to link the two resources: the equivalent would be https://w3id.org/fdof/ontology#isMetadataOf :
fdof:isMetadataOf rdf:type owl:ObjectProperty ;
                  rdfs:subPropertyOf gufo:historicallyDependsOn ;
                  rdfs:domain fdof:MetadataRecord ;
                  rdfs:range fdof:DigitalObject ;
                  dct:description "Identifies the fdof:DigitalObject the fdof:MetadataRecord is about."@en ;
                  rdfs:label "is metadata of"@en ,
                             "isMetadataOf"@en .

This seems fundamentally acceptable, except the domain and range may not be exactly right (they seem very close).

PROPOSED: we create a Subject Identification Element that uses that property to declare the subject IRI. Since a standalone field in CEDAR can’t hold a property identifier, we will need a metadata description element that contains the Field to make the user experience straightforward and yet computable.

This is not a ticket to change CEDAR per se, but it seemed the best place to manage the discussion by CEDAR users for this operational procedure. Perhaps CEDAR can agree to put the resulting element in its Community section for easy access.