OPT: sct_extract_metric: now possible to specify slices (not only range)
OPT: sct_extract_metric: now possible to specify slices (not only range)

Former-commit-id: 81aded0c662c2f911b7406f95864845f106edf7e [formerly 4a5e55ac6ac1e965bda02c64328aca93134d8f2b]
Former-commit-id: d941719ba0d9768658d1ca501f3e600c279d355b
Former-commit-id: 2523839ef469e56c1fe41ae76694eaa04ab6cfbb
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox