OPT: corrected the dimensions of the z pixels of the reference image
OPT: corrected the dimensions of the z pixels of the reference image

Former-commit-id: 39d8e4352eb37350c567d0dbab6ef403b1ab622b
Former-commit-id: dcd72750220cec943cceeba0bbd6824923fcc7cd
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox