Spatial smoothing in antsMotionCorr

[cookpa]

Hi everyone,

#1350 fixes a problem with antsMotionCorr that has some backwards compatibility implications.

Smoothing was being interpreted in mm units for affine and related transforms (eg, rigid), but in voxel units for deformable transforms.

To address this, #1350 makes the default unit voxels for all transforms, and accepts the "vox" or "mm" specification. This makes antsMotionCorr consistent with `antsRegistration.

To maintain consistency with the previous implementation, append "mm" to the smoothing specification for global transforms, eg -s 1x0mm.