Change RelatedArtifact to provide needed functionality

Update the RelatedArtifactType code system to what is in the comments from Brian Alper. Add an implementation note to the datatype that bidirectional relationships SHALL NOT be created via the combination of type code and the FHIR resource reference. (see description for more detail) Add a new element named targetClassifier 0..* of type CodeableConcept with an example binding to the CitationArtifactClassifier value set. Add a new element named resourceReference 0..1 of type Reference(Any) with an implementation note that the resource element and resourceReference element SHOULD be consistent. Remove url as it is redundant with document.url. Resources that are using RelatedArtifact will need to be changed if they have overlapping relationships that can be expressed using the new changes.