Use JSON-LD processor to generate RDF

[nichtich]

Change to_rdf_graph to not use rdflib_dumper.as_rdf_graph but generate JSON-LD and use a JSON-LD processor (e.g. PyLD) to generate RDF. This would also ensure that JSON-LD export does actually produce the same RDF as RDF export.

This would also solve #590 when in the JSON-LD context

"mapping_set_id": { "type": "@id" }

is changed to

"mapping_set_id": "@id",