Started adding support for the BAG dataset.

* [BAG,NEW] New module accessing the BAG dataset.
* [DBPEDIA,FIX] Rewrote the WHERE-clause of the find_dbpedia_agent/4 predicate, adepting to the improvements made to formulate_sparql/5.
* [DC,RDF_NAMESPACE,SC] Renamed namespace 'dcterms' to 'dc'.
* [RDF_NAMESPACE,EXT] Added namespaces for bag, bags, ogc, plod, prv.