URREF distribution IRI schema

[jdreo]

The default prefix in URREF is http://eturwg.c4i.gmu.edu/files/ontologies/URREF.owl (since commit 365e1f3).

However, this does not quite follow the convention of having prefixes without extension, like: http://eturwg.c4i.gmu.edu/files/ontologies/URREF#

But the IRI should also allow distribution of previous URREF versions.

I suggest implementing a naming schema that would use …/URREF as the latest version, and …/URREF/<version>/URREF as previous versions (e.g. …/URREF/4.0.0/URREF). So that we would have, for instance:

<http://eturwg.c4i.gmu.edu/files/ontologies/URREF> rdf:type owl:Ontology ;
    owl:versionIRI <http://eturwg.c4i.gmu.edu/files/ontologies/URREF/4.0.1/URREF> .

[jdreo]

This should of course be used on the website as well.