DEB: fixing edge effects when calculating spinal cord shape
DEB: fixing edge effects when calculating spinal cord shape

Former-commit-id: 018cca8e4298f83dab12a1195e8f1544cc8ad903
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox