OPT: add the flag -output-map to produce a metric map based on the metric value estimated and the fractional volumes of each label, useful to assess the estimation results and especially the partial volume effects.
OPT: add the flag -output-map to produce a metric map based on the metric value estimated and the fractional volumes of each label, useful to assess the estimation results and especially the partial volume effects.

Former-commit-id: 7d9f2a3c4813e56bdc48b8e426d7d898009064bc
Former-commit-id: 5fa88fcc7cf0600fb5fe00326f5cc60440f28ea4
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox