Support concordances

[nichtich]

A concordance is a set of mappings between two concept schemes, possibly enriched with some metadata (publisher, source...). JSKOS should support concordances as well. Some general notes:

• mappings and concordances can be related to each other with RDF properties dcterms:hasPart / dcterms:isPartOf.
• a mapping can be part of multiple concordances (can't it?)
• all mappings in a concordances should be from the same and to the same scheme

[nichtich]

Included in 0.0.18 but without integrity rule and no RDF property.