Bundles can't pass through csf

[samuelstjean]

On the left is the old version of the tracking mask for the the isbi phantom and on the right the new one (made using the phantomas_rois scripts, this is fibers.nii). It seems like the partial volume spheres are pure csf, and are not included in the tracking mask as shown on the right, while they seem to be included in the old version.

This reduces some bundles to 1 or 2 voxels of width, while the old version had more voxel and would allow passing through the spheres.