Add SKOS (saref-portal#67)

From saref-portal#67 (closed)

In SAREF4CITY, the accessibility of the event (s4city:hasAccesibility - contains a typo) is only provided as skos:Concept. A concept scheme should be provided or linked to.

Add skos:ConceptScheme

Changes have to be performed in the OWL code, TS, diagrams, documentation and tests.