OPT: sct_extract_metric: use matricial form to compute ML (without using lstsq function)
OPT: sct_extract_metric: use matricial form to compute ML (without using lstsq function)

Former-commit-id: 28278e1cf19c6caf57c81e345e5b714b95177ffa [formerly 22e76145dbd7c670d0b087b7cfce3a3c5debbbc8]
Former-commit-id: 6075f7d4822157093386ce716525ae78cf02c5eb
Former-commit-id: 53422a902e1537c7f7227088695f3349d0263937
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox