OPT: msct_pca: mean image easily available (not only mean vector)
OPT: msct_pca: mean image easily available (not only mean vector)

Former-commit-id: 8f16ffaf15c4da169346ef3f25ddcf9942a27719
Former-commit-id: a00db23eefc4f591fad8d853706abd6b458dbb31
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox