OPT: added param to compute model in command line with msct_multiatlas_seg
OPT: added param to compute model in command line with msct_multiatlas_seg

Former-commit-id: 9c8a17e002a33b091188f2f782819811cd1ab3a0
Former-commit-id: 63091d13eb8fe1df915078b1d9bbc752cb59f13c
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox