Persist virtual ontology network structures

If the OntoNet bundle goes down and back up, the mappings between ontology IDs and graphs are retrieved because they come from a stored graph, but any information on scopes and sessions goes away.

The old OWL API implementation was relying on a configuration file whose address was set in the ONManager component config. This is sub-optimal. To begin with, the scope/session structures should be stored in a graph, e.g. the same as the one storing ontologyID->graph mappings (to save space on *nix filesystems).

RDF graphs obtained by parsing external files could then become the override mechanism for creating new scopes/sessions instantly. To this end, the ontology network structures on a Sanbol installation must always be exportable to RDF.