NEW: compute MI (using function calc_MI originally from sct_label_vertebrae)
NEW: compute MI (using function calc_MI originally from sct_label_vertebrae)

Former-commit-id: 3e2dffcffa3cc78d477aa9c8695aaa5b06bdc112
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox