Refactored the sct_get_centerline file
Refactored the sct_get_centerline file

Comply to the pep8 style guide and removed dead code.


Former-commit-id: 3a119fed618673d9608c571b6c815100505dfb6d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox