BUG: Fix ImageMath warning about operation not found, which was still happening on some ops
because the warning was being printed before all possible operations were searched.

BUG: Fixed some typos in ImageMathHelper calls where dimension was set to 3 instead of 4

ENH: Added option in ImageMath to set nonzero background diffusivity when masking tensors.
Helps avoid annoying interpolation artifacts when warping tensor images.