Neper material ID start at 0 if a buffer layer is added
(run "neper -T -n 100 -tesrsize 320:320:320 -domain "cylinder(20,10)" -format vtk"),
otherwise at 1.
Hence, leave the IDs untouched and let the user change the
numbering (either by a constant offset or by re-numbering): "explicit is better than implicit"