Add dct info to ConceptScheme

[koenedaele]

We should be able to handle dct information for a ConceptScheme.

In iso-thes there's a mandatory dc(t):language attribute. This indicates what languages can be found in the thesaurus. Two possibilities: determine this automatically by looking at all languages present in the concepts and collections in the scheme or handle it manually.

Other attributes:

• dc:contributor
• dc:coverage
• dc:creator
• dct:created
• dct:modified
• dc:date
• dc:description (can also be mapped to rdfs:comment)
• dc:format
• dc:publisher
• dc:relation, dct:relation or a specialization
• dc:rights
• dc:source
• dc:subject
• dc:title (will just be handled by skos:prefLabel and should be mapped when in and exporting)

Parked for now

• dc:type (until there's a vocabulary of types everybody agrees on, and even then)