Interpolation and Smoke Velocity

Updated the smoke2exr coordinate calclation to include interpolation between slices. Still not perfect (there's some banding still evident) but much better than without interpolation. Here's an example using the smoke velocity extracted from the smoke domain point cache using Smoke2EXR. Working on a number of tutorials to cover this and how to put together the volumetric distortion examples (this was slightly cheating since it's generated from smoke emitted from a particle system, rather than being generated procedurally).