BUG: get_centerline: fixed #550
BUG: get_centerline: fixed #550

Former-commit-id: 2ab46313d9c79574f151da66ff1aa1cb2b4b1312 [formerly 346b5ba46d2de0487b52a13134e47097ca2aa827]
Former-commit-id: 32aef01d1328344911b32495e38413b7120d4a11
Former-commit-id: a49394e3e86ca33975794bbb8660844096c0ccd8
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox