OPT: changed behaviour of straightening for low number of slices
OPT: changed behaviour of straightening for low number of slices

Former-commit-id: 6628f6918e1263282e4d12e0334dda24c319d4f8
Former-commit-id: 3cb60bb60550a007c6a8f435577e6493bf1bdf25
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox