sct_maths: Convert usage of convert_list_str to use list_type (#2901)
sct_maths: Convert usage of convert_list_str to use list_type (#2901)

Uses argparse's `type=` instead of a postprocessing function. Also, this PR clarifies argument descriptions.
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox