sync_with_triplestore and sync_with_rdf_file

[iosonopersia]

A sync_with_triplestore method is contained inside the GraphSet class, but it's missing from MetadataSet.
Moreover, a sync_with_rdf_file variant should be added to both classes, enabling the user to synchronize both
a triplestore and an RDF file with the user's GraphSet/MetadataSet: these methods should be able to import the entities
that contain a reference to an entity which is going to be deleted and should also remove those references.

Maybe these methods should be called something like propagate_deletions_to_triplestore and propagate_deletions_to_rdf_file.

These methods should not be present inside ProvSet, since provenance entities cannot be deleted!

[iosonopersia]

Method sync_with_triplestore was renamed to remove_orphans_from_triplestore within the commit be91122 .