BUG: get_centerline: string --> str
BUG: get_centerline: string --> str

Former-commit-id: 54969529bf5c8a33e478cfe45ee3f8c66d7f4dad [formerly 1622ce5eb7d21343bc5629b1c9d9a7e5dbba7530]
Former-commit-id: 5ca4cd1ab99f0f8e19c0d208208eb036f8b37cd5
Former-commit-id: 518e0590808fca51ebd34eadfce123f39eb1dc90
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox