dcat:themeTaxonomy - check constraints #119

In DCAT v1 the property dcat:themeTaxonomy is axiomatized

dcat:themeTaxonomy
  rdf:type owl:ObjectProperty ;
  rdfs:domain dcat:Catalog ;
  rdfs:range skos:ConceptScheme ;
.

1. Verify that the range is appropriate and necessary
2. Verify that the domain is appropriate and necessary (see Review global domain axioms on dcat properties #110)
3. Consider whether any guarded constraints (using owl:Restriction) should be introduced (see Use owl:Restriction constraints to bind DC properties to DCAT classes  #105)