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.
This seems to cause some trouble in many places that are expecting SMWExpData but now getting an array of SMWExpData.

For example getDataItemExpElement()

I suspect that this is also causing the test failures.