NEW: msct_pca : colour slices by levels when plotting modes
NEW: msct_pca : colour slices by levels when plotting modes

Former-commit-id: 46eee33b21a13ed208193e106f1511654364d0b6
Former-commit-id: a06533465647917066590d9d9f63388714593f8f
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox