OPT: sct_extract_metric: added robust ML estimation followed by WA after correcting for PVA (mlwa)
OPT: sct_extract_metric: added robust ML estimation followed by WA after correcting for PVA (mlwa)

Former-commit-id: b1de0a2189266d536c83d9d80d27fec0bbed07cc [formerly dee2a51ee6e10dd1186723f122ca2b86fb12d2dd]
Former-commit-id: 05f4d79472a7ca4aacdf9924359776aaba40b2bb
Former-commit-id: 2778b7f88ccb3e5d8cf2ae1d4223024ca48071a9
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox