blank nodes for distribution, dataservice and catalogrecord

[NielsHoffmann]

I'm testing the transformation to geodcat ap for the 3.0.0 pilot.

With the sample iso records i'm working with (valid records from the dutch NGR) CatalogRecord, Distribution and Dataservice result in blank node statements in the rdf output.
While technically probably correct, a lot of tooling can't handle blank nodes easily so the connection between the Dataset and it's Dataservice, Distribution and CatalogRecord is lost.
I think it would be beneficial to explicitly serialize the dcat:distibution, dcat:servesDataset and dcat:dataset properties.