fix ischeme vtk for point*tensor reference
fix ischeme vtk for point*tensor reference

The `getischeme_vtk` method of the `TensorReference` generates integration
points with incorrect order if `ref1` has dimension zero.  Such references
originate for example from the boundary in dimension zero of a structured
topology.  Since `ref1` has no contribution to the integration points anyhow,
this patch fixes this issue by forwarding `getischeme_vtk` to `ref2`.

joostvanzwieten/nutils,CVerhoosel/nutils,wijnandhoitinga/nutils,nutils/nutils