[vtkusers] Create its own dataset

[François Bertel]

vtkUnstructuredGrid is the right dataset (and the only one) that can 
handle tetrahedra. All standard filters can be used on
vtkUnstructuredGrid (like vtkContourFilter)

field data on center of tetra can be set as cell data.
field data on nodes of tetra can be set as point data.

field data on intermediate topology like faces cannot be
handled directly.  The common workaround is to pass such field as cell 
data and apply a vtkGeometryFilter on the UnstructuredGrid. Since the 
output is a surface, the data appear only on the faces.

-- 
François Bertel, PhD  | Kitware Inc. Suite 204
1 (518) 371 3971 x113 | 28 Corporate Drive
                       | Clifton Park NY 12065, USA