Fixes RDF export of subobjects

Adds SemanticData from subobjects to the RDF export fixing https://bugzilla.wikimedia.org/show_bug.cgi?id=48708

Note: This is a partial integration of the patch https://gerrit.wikimedia.org/r/#/c/70047/4
Credits to Dbeeson



Code changes in short:
The Export code for RDF now understands that SemanticData has recursive information about subobjects and tries to iterate over them to add to the RDF export.
@nischayn22 Preliminary note #361 is moving this part into its own component (to make it easier to test SemanticData -> Turtle encoding); once #361 is merged this part (uses the same method naming) should be moved.