REF: utils/check_if_3d: now returns True/False statement instead of quitting
REF: utils/check_if_3d: now returns True/False statement instead of quitting

Former-commit-id: e41329dff81947f9ba00c0795eee78542caa9e24
Former-commit-id: a8a9924d249cbaed5d47815c5bc206daaf5f375b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox