REF: Make 'step' mandatory. See issue #1034
REF: Make 'step' mandatory. See issue #1034

Former-commit-id: e2de43324daf8c216da82bc3df70c7cf4e592066
Former-commit-id: baca0e82ddfc185987ec9eee1db890eecb68077b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox