[sct_segment_graymatter] correct background value
[sct_segment_graymatter] correct background value

Bug was that background value of WM segmentation was not null, inducing a bias when computing WM CSA. 
--> Bug occurred during the interpolation of WM segmentation back into the native image space.
--> Corrected by setting negative values to zero right after the label fusion (only very few voxels had negative values).

Former-commit-id: eb210f9e95b7d2c338b4cd945a6daf55b0284f7c
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox