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: 7b9fa579b4adaa8028a7a3abb0fd67cbf7108479 [formerly 791cc8e3e608c0c436a9e919c68384ead7c5ac60]
Former-commit-id: 094c420f4446e5445106477f4808c69588c38f86
Former-commit-id: 844413e1b480edd430130aa53a59c9c138518c59
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox