Change contributor and provider values to IRIs

[balhoff]

We have long had comments pointing out that contributor and provider values are strings representing IRIs:

go-shapes/shapes/go-cam-shapes.shex

Lines 120 to 124 in f5b117b

	contributor: xsd:string +; #TODO would be better as an IRI
	modification_date: xsd:string {1}; #Use ISO 8601 as a standard.
	creation_date: xsd:string {0,1}; #Use ISO 8601 as a standard.
	import_date: xsd:string {0,1}; #Use ISO 8601 as a standard.
	provided_by: xsd:string +; #TODO would be better as an IRI

When we implemented model copy in Minerva, we used an IRI value for the prov:wasDerivedFrom property. This doesn't seem to cause any problems in Noctua. Is there anything stopping us from changing these to IRIs? (besides migrating the data and updating Minerva)

IRI values are much better for triplestore queries, such as finding all models contributed to by a particular person.

[kltm]

As long as things are going in and out of Noctua as they currently are (i.e. "typeless" JSON), there shouldn't be any issue with things built on top of Minerva. That said, there are API pieces that currently make use of the sparql endpoints, so it's possible that a change in data type could make a difference. Tagging @sierra-moxon and @dustine32 for feedback.