OPT: target input is now optional (allow to ably compute a model)
OPT: target input is now optional (allow to ably compute a model)

Former-commit-id: accebcbee3f8d4ddb0b784eb570639472a8e1061
Former-commit-id: 528fb43835ef9500d67e638ae1e0e9a95d3ff2b0
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox