BUG: fixed computation of error in straightening
BUG: fixed computation of error in straightening

Former-commit-id: f8f31a1743e69adfaebb746c190e8fdedae53ddf
Former-commit-id: a8a46dca119301197fdc11596087da5207b1ca12
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox