SpatialReference inherits SemanticEntity

[niniemann]

At some point I needed a semantic description of the spatial reference frames that are present in sempr; knowledge about the parent of a LocalCS, to be precise.

To achieve this I changed SpatialReference to inherit SemanticEntity instead of Entity. That way I could easily register the parent_ member of LocalCS as a property to be exported as an RDF triple. I also added a property for the (semantic) type of the object (--> "\<sempr:LocalCS>") using a static const std::string variable -- which feels a bit hacky.

• Is inheriting from SemanticEntity okay for SpatialReference?
• Do you have any suggestions for improvements?
• Should we consider to also let other classes inherit from SemanticEntity? E.g., Geometry could export the id of the assigned SpatialReference.

[niniemann]

More than stale. (Development proceeded in an internal git)