Implement new datamodel from RDF

[henhuy]

Additionally to datasets stored into django models and queried via django ORM, we want to SPARQL data from OEP which is first mapped into RDF format and then made available via RDF endpoint(s).

In order to test/implement this, following steps must be done:

• Add flag (from_rdf or similar) to source model
• RDF Dataset does not store data in DB, only scenario ID, RDF endpoint and source RDF
• rewrite get_filters to get filters from RDF endpoint in case of RDF source
• rewrite get_scalar_data to query via SPARQL at endpoint, instead of DB query
• Setup RDF endpoint from files for testing via https://github.com/vemonet/rdflib-endpoint
• get example RDF files (from ADEL)