Property to link to the actual implementation artefacts in a profile description

[dr-shorthair]

On the basis of a small test of the Profile Description ontology
https://github.com/w3c/dxwg/blob/profiledesc-examples-simon/profiledesc/examples/adms.ttl
I think we need a property to carry explicit links from the ImplResourceDesc (an association class) and the actual implementation resource.

I suggest calling this either pd:artefact or pd:implementationResource with the RDF definition

pd:artefact
  rdf:type owl:ObjectProperty ;
  rdfs:comment "The artefact (resource) linked to a profile through an Implementation Resource Descriptor" ;
  rdfs:domain pd:ImplResourceDesc ;
  rdfs:label "artefact (resource) " ;
  skos:editorialNote "A property to link from a pd:ImplResourceDesc to the actual implementation resource. " ;
.

Related to #323 and #355