OPT: do not use z-regularization on 2D data
OPT: do not use z-regularization on 2D data

Former-commit-id: f83105992c869309e785321926404d8a2b24aa31
Former-commit-id: a645fe821a51e71b47d7fbdbe3709b65a858f368
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox