[vtkusers] new vtkCurvatures class added to Graphics

[John Biddiscombe]

I've just checked in a new class call vtkCurvatures which computes the
Gaussian or Mean curvature of a polydata object.
In Graphics/Testing/Tcl is a new test call TestCurvatures.tcl which
illustrates how to use the class.

Many thanks to Philip Batchelor <philipp.batchelor at kcl.ac.uk> for
contributing the class and Andrew J. P. Maclean <a.maclean at usyd.edu.au> for
fixing various bits and pieces.

I've tested it fairly thouroughly against a range of polydata datasets and
put some checks in to prevent floating point errors when degenerate data
and/or other dubious polygons are supplied - but there are almost certainly
going to be a few possible cases which cause trouble. Please send any bug
reports or improvements to the list, one of the authors or myself.

(Polydata with discontinuities or other sharp features can cause very
(unrealistically) large scalar values to be generated - perhaps some sort of
output clamping value should be added?)

JB