REF: refactored straightening
REF: refactored straightening

Former-commit-id: e6fbdbc604372d7802a9612d8a573449e3ab96b4
Former-commit-id: 34890d3739e7fffcdc6298d7439f635396802162
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox