Extracting metrics into the CSF, WM and GM with sct_estimate_MAP_tracts

Enable to extract metrics into the CSF, WM and GM with sct_estimate_MAP_tracts.py.
Also add the true segmentation of CSF and rename the segmentation of CSF+cord "MNI-Poly-AMU_CSF_and_cord.nii.gz" to avoid future mistakes.
JUST BE CAREFUL: if the new version of the script estimating metrics ("sct_estimate_tracts.py") still doesn't allow to extract metrics into CSF, WM and GM, it might crash after merging this branch as the file "list.txt" in the atlas of the toolbox of this branch includes labels 30 (cerebrospinal fluid), 31 (gray matter) and 32 (white matter).