cubicweb-skos #5313855 [rdfio] graph add method should work on string (not on URIRef) [open]

For consistency with other methods (objects, subjects), add method should accept strings as parameters, and not URIRef instance.

For example we can write:

graph.objects(predicate_uri='http://www.w3.org/ns/duv#provides')

but we cannot write:

graph.add("http://example.org/agent/Bob", "http://www.w3.org/1999/02/22-rdf-syntax-ns#type", "http://xmlns.com/foaf/0.1/Agent")

Instead we must write the following (and this is painful):

graph.add(graph.uri("http://example.org/agent/Bob"), graph.uri("http://www.w3.org/1999/02/22-rdf-syntax-ns#type"), graph.uri("http://xmlns.com/foaf/0.1/Agent"))
prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>