Add a mask for the QSpace conversion

This MR adds an input for a mask image to discard some pixels for the QSpace conversion. Pixels with non-zero value in the mask are discarded (Is this convention OK?). The mask is expected to have the size of the images.

The mask is handled during the computation of the histogram look-up table.

As it is now, it is mutually exclusive with the use of binning and median filter. I can make the mask and image binning working together if needed, but I see little interest in it as the next step is a histogram. Let me know if you want it.