Add ontology versioning support

[gtfierro]

    g = Graph(graph_name=URIRef("urn:example"), load_brick=True)
    graph_name = g.value(predicate=A, object=OWL.Ontology)
    assert graph_name == URIRef("urn:example")
    imports = list(g.objects(subject=graph_name, predicate=OWL.imports))
    assert len(imports) == 1