OPT: converted sct_register_pipeline to multiprocessing for straightening
OPT: converted sct_register_pipeline to multiprocessing for straightening

Former-commit-id: cff5e0cddd91a145ba3e1561422cd0214a9bde6a [formerly b38430f14886c8e59180a64974061184d21ec10a]
Former-commit-id: 956f611ff11144a7e5a50edc607078f2ef27653f
Former-commit-id: bb883a1a9631cc3405f656bce845c526825acaf2
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox