Speed-up and improved meaning of index image

Added SamplingRate parameter. Now this filter can run faster, by initially sampling the colors at a coarser level. For a sampling rate of (5,5,1), i.e. 25 fewer voxels to build the octree, I got an overall factor of 10-15 speed-up.

Added SortIndexByLuminance parameter. The "index-image" viewed as a greyscale image, is usually quite arbitrary, accentuating contrast where none can be perceived in the original color image. To make the index image more meaningful (e.g. for image segmentation operating on scalar images), we sort the mean colors by luminance and re-map the indices accordingly. This option does not introduce any computational complexity and has no impact on actual colors in the lookup table (only their order).