OPT: now enables to extract metric from csf, white matter and gray matter (comments and variable designations were also updated to clarify the code)
OPT: update files in order to be able to extract metrics from csf, white matter and gray matter with sct_estimate_MAP_tracts
BUG: fixed a small bug in help display
Merge branch 'sct_estimate_MAP_tracts' of github.com:slevyrosetti/spinalcordtoolbox into sct_estimate_MAP_tracts