OPT: compute_ernst_angle: added example value for T1 in the gray matter and changed default value for WM
OPT: compute_ernst_angle: added example value for T1 in the gray matter and changed default value for WM

Former-commit-id: 921cd39ddd2be07192bdca1ad3d44bd3f3753141
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox