- getting data out of VTK is a functionality of the VTK class. Need to
discuss whether the VTK class should also provide grid and size (works
only for rectilinear grid, so hiding VTK type is not possible
anymore), but the current situation requires the Geom class to rely on
'internals' of the VTK class
- header is specific to *.geom format, not to Geom objects.