OPT: extract_metric: new method "max" used for extracting CSA on interpolated CSA volumes
OPT: extract_metric: new method "max" used for extracting CSA on interpolated CSA volumes

Former-commit-id: c080aa93c6de38df40d404e2bc59765c1794f90e
Former-commit-id: 9a8bb3443a9d37da575e5a56a725ae0d56919db8
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox