rdfs prefix
removed unused variable
SSN and DUL prefixes
ssn:observedProperty
measurements described with SSN (fixes #73)
Merge branch 'ssn' of https://github.com/enviroCar/enviroCar-server into ssn

* 'ssn' of https://github.com/enviroCar/enviroCar-server:
  DBPedia links added
  Fix of import on DUL