[vtkusers] smoothing / decimation / isocontours / distance between surfaces

[John Biddiscombe]

> also, i was thinking maybe raycasting from the top and then the bottom
might
> work? like by marking the first point it hits at a certain level... but it
> seems that all the ray casting functionality is now Volume specific. maybe
> vtkVolumeRayCastIsosurfaceFunction might be useful somehow?

Usually with raycasting in this type of problem, the question is "in which
direction do I fire the ray", if the surface is not flat, the opposite side
may not be obvious. Since the cloud base and top "ought" to be at least
vaguely horizontally aligned, you may get away with vertical rays to get an
estimate rather than rays along the surface normal.

JB