OPT: updated Image class so it is possible to create an empty image
OPT: updated Image class so it is possible to create an empty image

Former-commit-id: 4d2cd6b7826732f8de7df22c1cec9e365b5b7f64 [formerly cab2a16d6afa11d73868c47fc7031900ac8ea2ef]
Former-commit-id: f50e2883c15ae067aed7f1c325fbe7e4885048ef
Former-commit-id: 1a88bbe0e389fac73d1e18a85c154af2c47e5f8b
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox