Align metadata schema with DCAT v2

[philipashlock]

DCAT 2 has not been finalized quite yet, but it is scheduled for publication Q2 2019 (the Data Exchange Working Group Charter goes until June 30 2019).

The current draft can be seen at: https://w3c.github.io/dxwg/dcat/

The current version of DCAT used as the basis for Project Open Metadata Schema v1.1 is the 2014 release.