Allow output of number of changed pixels/voxels during filters execution.

I have a suggestion for a small change - but one that would involve many filters. As a starting point, when we clean up 2D data using TSL, after each operation it reports on the fraction of the pixels that were changed. It gives you some idea of how much you are altering the raw data. My students are wondering, after a dilation, fill bad data, or similar operation, what fraction of the voxels were changed by the filter? If the filters reported this, it would be very useful information.