cubicweb-dataio #3774876 Add a stepsize to xy import [validation pending]

The rdf import command should have a stepsize option to split import of huge files.

prioritynormal
typebug
done in0.5.0
load0.200
load left0.000
closed by#f1cb6a5e935c [ccplugin] Add a stepsize to xy import, closes #3774876
patch[ccplugin] Add a stepsize to xy import, closes #3774876 [applied]