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

Former-commit-id: 5f728103e99f2aafcd01ea6e0ad105dc05b9b6b5 [formerly 46eee33b21a13ed208193e106f1511654364d0b6]
Former-commit-id: a06533465647917066590d9d9f63388714593f8f
Former-commit-id: c21cd9e9ff00891843a9e81942c93c074c7fe1be
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox