BUG: get_centerline: fixed issue related to the confusion with input_image and fname_in
BUG: get_centerline: fixed issue related to the confusion with input_image and fname_in

Former-commit-id: 8d9a6f780b51b689f56fc26d12bc48485163687b [formerly 99b552e93467cd1d9991c1a5334e82e363a092d7]
Former-commit-id: 89189bfe861fc2035ae4ce23fa7a54b0e6ad08c4
Former-commit-id: 5531430341b3230a2f5c09b642dbb891d5b7a5c8
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox