Fix the relationship between a file containing a multi-graph serialization and the loaded graphs.

[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.

[Anniepoo]

Jan warned me about this a while back. That graph originally meant 'resource the triple came from'. People started using it for other things, and then then wanted to know provenance again, and triples become quads, become 5-tuples, and so on.