Some notion of semantic distance between the two concepts. Currently, it returns Double.MAX_VALUE if any of the
concepts is null or unknown to the ontology or if they are completely unrelated (they don't share any ancestor in
common, something that cannot happen in RDF-based ontologies, all concepts derive from 'Class'). If the two
concepts are the same, the distance is 0. If one is a parent of the other, the distance is 0.5 (nevermind the
distance). Otherwise the distance is 0.5 plus 0.1 for each level up until a common ancestor is found.