INST: now checking if FSL is working, and THEN if it is installed (only if not working)
INST: now checking if FSL is working, and THEN if it is installed (only if not working)

Former-commit-id: 4b5e9e59f2fed5098a931ed7a400f5481a7693ce
Former-commit-id: cce9775843c22dc100b8e2d8a8c7907d048e9cc2
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox