ENH: Allow smoothing to be specified in mm or vox

[cookpa]

Previously, affine transforms used the registration method default (mm) while deformable transforms used voxels.

Made them consistent with antsRegistration, allowing AxBxCmm or AxBxCvox specification.

[cookpa]

This is for antsMotionCorr, I probably should have clarified in the comment

[gdevenyi]

To clarify, this means antsMotionCorr smoothing options were in mm unless otherwise specified, which is opposite to antsRegistration?

     -s, --smoothing-sigmas MxNxO...
          Specify the sigma of gaussian smoothing at each level. Units are given in terms 
          of voxels ('vox') or physical spacing ('mm'). Example usage is '4x2x1mm' and 
          '4x2x1vox' where no units implies voxel spacing. 

Will the default remain mm?

[cookpa]

The default is now voxels, consistent with antsRegistration. It was inconsistent before in the code, an affine transform specified smoothing in mm, and SyN smoothing was in voxels.

A default of mm is more backwards compatible for most use cases, but then we're committing to being different from antsRegistration, which is its own problem.