OPT: improved speed of straightening
OPT: improved speed of straightening

Former-commit-id: 0785d09349de0464fdfd335f861a2e86369b7173
Former-commit-id: 5d20b5cf1069c142508139228adfaaade37d49ba
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox