SES-1850 SES-2319 : Programmatic access to vocabulary IRIs, mainly for the purpose of interning references to common datatype IRIs

Add Vocabulary interface for programmatic access to IRIs. Also provides programmatic access to the Namespace object, to get the most common prefix and base IRI for each vocabulary.

This is required before being able to efficiently cache all of the IRI objects that are being created in our vocabularies, in order to reduce memory usage for large parsers/models/repositories.

Add SimpleVocabulary as an implementation of Vocabulary.

SES-2319 : Reuse IRIs for commonly used datatypes by checking known vocabulary IRIs for the IRIs and then replacing the users references with those that are known to be authoritative.

NOTE that this is not yet complete, as it does not provide access to all of the vocabularies, but two, DC and XMLSchema are done to provide a proof of concept.