OPT: add flag -output-type and -overwrite like in sct_extract_metric (only for mean CSA and volume estimations)
OPT: add flag -output-type and -overwrite like in sct_extract_metric (only for mean CSA and volume estimations)

Former-commit-id: 154ed530cb6b2ec7c87643ae634cf32f2c0a0d9c
Former-commit-id: 4e9b25537a0d7646423ff2fb7cb9b61af2594a75
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox