[WIP] #25 basic sparql filters (incomplete)

with some namespaces allowed
fix lost namespaces in data model #25

sparql filters can now be used as long as the used prefixes are
declared in the data model (not (only) the mapping model)
clean up
Merge pull request #27 from janpolowinski/refactor-and-add-sparql-filtering

Refactor and add sparql filtering