Integrate match certainty in RDF graph

[rue-a]

For each mapping (exact_match, close_match, narrow_match, broad_match) from a harmonized concept/class to an external concept/class, users of the package have to give a match certainty, reaching from 1 - uncertain, to 3 - very certain. This match certainty is currently not integrated in the RDF graph that is generated with export_as_rdf().

I can imagine two ways to do this, one would be using RDF reification, the other one using singleton properties, where I tend towards the use of singleton properties.