OPT: msct_pca: modes visualization
OPT: msct_pca: modes visualization

Former-commit-id: 82ace373179fc925da707f90c8b6b999049d9705
Former-commit-id: 53bed0cd384a189f166248c335044fe72bd281ff
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox