OPT: maths: smooth now gets sigmas in mm (before it was voxel)
OPT: maths: smooth now gets sigmas in mm (before it was voxel)

3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox