OPT: straighten_spinalcord: add ofolder flag for dealing with multithreading
OPT: straighten_spinalcord: add ofolder flag for dealing with multithreading

Former-commit-id: 30c1b49116daa9604435662b6a9469a14a7745a5
Former-commit-id: 8e8819d395f37480112fc23b485fc49c149c46b8
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox