There is a singularity when v = 0. This patch checks for v = 0 and
replaces it with a small number, .05.

A new ivar DeltaV0 can change this value.

This bug was noticed when running the VTK wiki example:
Cxx/GeometricObjects/ParametricObjectsDemo2. While rotating the camera
for the Kuen surface, the center of rotation appeared off. The
z-coordinate was NAN for points when v == 0.

Also, the ordering of triangles was incorrect.