Many additions for supperting / performing RDF(S) entailment.

* [DB_EXT,FIX] Declared assert_novel/1 as a meta-predicate.
* [MODEL_THEORY,NEW] New module implementing classic model theory.
** A satisfaction function for FOL logical constants and quantifiers.
** An interpretation function for n-ary first-order predicates.
** An assignment function for first-order variables.
* [RDF_ENTAILMENT,NEW] New module implementing RDF entailment by syntactic means.
** Predicate terms are individuals of rdf:Property.
** RDF vocabulary axiomatic triples.
** Some typed literals are individuals of rdf:XMLLiteral.
* [RDF_GRAPH,SC] Remove rdf_clear/1. Use rdf_db:rdf_unload_graph/1 instead.
* [RDF_GRAPH,NEW] Added rdf_graph_merge/2 for merging an arbitrary number of graphs into a new one. This standardizes the blank nodes apart.
* [RDF_GRAPH,SC] Removed rdf_copy_graph/2 and rdf_copy_triples/5. Now superseded by rdf_graph_merge/2.
* [RDF_GRAPH,NEW] Added rdf_is_graph_[proper_]instance_of/2 for checking whether a graph is a (proper) instance of another one. A test predicate is included.
* [RDF_MODEL_THEORY.NEW] New module implementing RDF model theory as used in RDF-SEMANTICS.
* [RDFS_ENTAILMENT,NEW] New module implementing RDF entailment by syntactic means. This is difficult!
** Infer rdf:type triples from rdfs:domain and rdf:range triples.
** Infer individuals of rdf:Property from the RDFS subproperty hierarchy.
** Use the RDFS subproperty hierarchy to infer arbitrary triples.