Finalized RDF model theory for simple entailment.

* [RDF_MODEL_THEORY,NEW] Added predicates for handling models:
** clear_model/0.
** print_model/0.
** Assert objects in pairs that IEXT maps to as resources.
* [RDF_MODEL_THEORY,EXT] Finalized the interpretation function:
** URI references that use namespace abbreviation are now expanded to full URIs.
** Allow Prolog variables to be used in triples. These will be mapped to blank nodes. The assignment function is used for this.
** Many small fixes (type checking, variable instantiation, assignment pairs must be functional, added documentation).
1. Added axiomatic RDF materialization module.
2. Added old implementation of Doyle's TMS.

* [RDF_AXIOM,NEW] New modules for the axiomatic materialization of RDFS graphs.
** Added all RDF axioms (from RDF-SEMANTICS).
** Added all RDFS standard axioms (from RDF-SEMANTICS).
** Added RDFS standards test derivations (from RDF-SEMANTICS).
* [DOYLE,NEW] Added an implementation of Doyle's original 1979 TMS (plus the paper).