Merge branch 'master' into dev

* master:
  ssn phenomenon linking Fixes #74
  Sensor SSN linker added.
  dbpedia linking for phenomenons Fixes #81
  moved DUL & SSN to vocab package
  measurements described with SSN (fixes #73)
  ssn:observedProperty
  SSN and DUL prefixes
  removed unused variable
  rdfs prefix
  DBPedia links added
  Fix of import on DUL
  Added licencse headers
  removed unused constants
  Added DUL and SSN ontologies
  Added DUL ontology resources and properties.
  link all measurements for a track
  Added SSN resources and properties
  SSN linkers
  switched deployments to https