SPARQL support: federate data and ontology

[kysrpex]

In issue #209, a basic API for SPARQL queries were implemented. However, in order for the SPARQL queries to be efficient, they should be run against the backend. Therefore, to obtain meaningful results, the ontologies installed in OSP-core should be somehow present in the backend. This creates the need of an ontology management and versioning tool on the wrapper for such backend, and thus a change of the wrapper API.

Another option to explore is to see if federated SPARQL queries are possible with rdflib. That would be ideal, so that the namespace registry (containing the ontology) and the backend could just be federated and no transfer would be needed.