[Semweb] Support for loading RDF data in datasets.

[wouterbeek]

When loading RDF data from serialization formats supporting RDF datasets (Trig, N-Quads), some RDF triples belong to a specified RDF graph or to the default graph (if not graph is specified).

Currently, the relationships between (1) the file containing the multi-graph serialization, and (2) the graphs imported from that file is not accessible via rdf_graph_property/2.

For example:

?- rdf_graph(G), !, rdf_graph_property(G,P).
G = 'http://www.lexvo.org/data/term/kir/%D0%B8%D1%81%D0%BF%D0%B0%D0%BD%D1%87%D0%B0',
P = hash('70ecb85395368b8e6b90a29bcad0a417') ;
G = 'http://www.lexvo.org/data/term/kir/%D0%B8%D1%81%D0%BF%D0%B0%D0%BD%D1%87%D0%B0',
P = modified(true) ;
G = 'http://www.lexvo.org/data/term/kir/%D0%B8%D1%81%D0%BF%D0%B0%D0%BD%D1%87%D0%B0',
P = triples(7).

The source and source_last_modified properties are missing.

[wouterbeek]

Moved this to the Semweb repo.