Align with XSKOS

[nichtich]

XSKOS, published by Data Documentation Initiative (DDI) extends SKOS with several useful features. JSON-LD context documents of JSKOS should provide mapping to XSKOS where approriate. Some candidates:

• ConceptAssociation and Correspondence seem to correspond to ConceptMapping and Concordance
• xskos:covers is like JSKOS field subject
• xskos:next and xskos:previous (sub-properties of skos:related) are like JSKOS fields next and previous
• ...

[nichtich]

Since 0.1.2 XSKOS is included in the (non-normative) context document but not all properties.