cubicweb-datacat #6822234 dcat-ap compliance [validation pending]

add missing fields so that dataset are valide according to https://joinup.ec.europa.eu/community/semic/news/open-data-support-releases-dcat-ap-validator

prioritynormal
typeenhancement
done in0.4.0
load left0.000
closed by<not specified>
patch[schema] Add some missing pieces for DCAT-AP [applied]