OPT: new way to run external processes
OPT: new way to run external processes

Former-commit-id: a087e698f9a81399401f0eb10b3e42575385235b
Former-commit-id: d2e9cd547ff18dcb211ce494a5dab05d90cb5b1f
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox