BUG: straighten_spinalcord: major refactoring. Now using analytical solution for computing labels (issue #722).
BUG: straighten_spinalcord: major refactoring. Now using analytical solution for computing labels (issue #722).

Former-commit-id: 3338df54f7d87258b90738119f7f481bcaf4da4c [formerly 6c38793ea48099e793e5184eb1d51aeda90fae41]
Former-commit-id: 68dbfb87dabeeee269472905e553a05201ee8d8d
Former-commit-id: c4f16ddd0dc7a9f8afac9036a377d868862e0a6d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox