[vtkusers] Design Reationale - Removal of vtkStrcuturedData (obsolete?)

[Chih-Wei Chiu]

The book "The Viualization Toolkit: An Object-Oriented Approach to 3D
Graphics (1996)", must be too old. I don't know which version of VTK it is
based, but it must be old for sure.

In Figure 5-11 (p.128), it shows the vtkStructuredGrid class inherits from
vtkPointSet and vtkStructuredData which I supposed to be obsolete, because
it's missing from the datset class hierarchy since VTK 3.2:

http://public.kitware.com/VTK/doc/release/3.2/html/classvtkDataSet.html

I wonder why it's removed and why new vtkStructuredGrid does not inherit
from vtkImageData.
--
Chih-Wei Chiu <http://www.csie.nctu.edu.tw/~jwchiu>
Computer Graphics and Geometry Modeling Laboratory,
Computer Science and Information Engineering Department,
National Chiao-Tung University, Taiwan