cubicweb-frbr #3774745 Add XY for RDF import [validation pending]

We should define an XY file for importing RDF data using Dataio cube.

prioritynormal
typebug
done in0.1.0
load0.500
load left0.000
closed by#febb62bad84c [xy] Add XY for RDF import, closes #3774745
patch[xy] Add XY for RDF import, closes #3774745 [applied]