image_to_mask feature in math-tools/common_tools possibly already somewhere else

[Gregoire-V]

I added a feature image_to_mask in July, which converts an image (either scalar or vectorial) into a mask. I put it in math-tools/common_tools. In this code, the output mask is 0 in the voxels where the input image is 0 (or a null vector) and 1 elsewhere. But I have the feeling that this feature already exists inside segmentation/basic_tools/thr_image (but maybe here, that doesn't work for vectorial images?), or segmentation/basic_tools/mask_image. As I don't understand well what the cxx files inside these two folders do, I'm not sure that it's the case. If so, maybe we should remove image_to_mask in math-tools/common_tools?