New command testing_diff_mask

[Lestropie]

In trying to sort out #2789, there's a number of places where there is a desire to compare two masks, but to allow some amount of tolerance in the comparison of those masks. This is done my running both through a smoothing filter.

A better solution would be to have a command testing_diff_mask, which has a command-line option permitting a tolerance of distance mismatch between the two images.

I'm intuiting that in the context of binary images, the should be a set of basic image processing steps to do the trick:

• Any voxels present in image A that is absent from image B even after N dilations
• Any voxels absent from image A that are present in ~(~B dilated N times)