OPT: straighten_spinalcord: fixed #606 and crop image before calculating BSpline
OPT: straighten_spinalcord: fixed #606 and crop image before calculating BSpline

Former-commit-id: 791cc8e3e608c0c436a9e919c68384ead7c5ac60
Former-commit-id: 094c420f4446e5445106477f4808c69588c38f86
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox