NEW: msct_gmseg_utils: LOOCV by slice including levels
NEW: msct_gmseg_utils: LOOCV by slice including levels

Former-commit-id: 4bbf887f4204a180e4226c0c1461d810ca9f2bfa
Former-commit-id: 9443fa7b748e3f7452002c88f53344fa784f4658
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox