OPT: improved pipeline for straightening
OPT: improved pipeline for straightening

Former-commit-id: cef51f1557bb15166cbeb438d1a83c496fdfa7e3
Former-commit-id: da2e2823a71c5cb99484f72c8893b738789013ef
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox