[vtkusers] Change CellType

[Martin Baumann]

Hi,

it seems as if files written with EnSightWriter only work if the 
CellTypes are certain ones.
For example an vtkunstructuredGrid with CellType 11 (VTK_VOXEL) doesn't 
work in EnSight where the
same data but represented with CellType 12 (VTK_HEXAHEDRON) did work!

I tried to find a way to change CellTypes. In the mentioned example I 
did this by hand.
Is there a function that converts CellTypes? I guess the problem comes 
from the different numbering.
It might be enough to change the numbering.

Thanks, M.B.