ENH: bring back option to have uniform density at surface

gyral_structure/scripts/gs_fit

@@ -32,7 +32,8 @@ parser.add_argument('-s2', '--second_rbf', default=7., type=float,
                     help='RBF Dipole size during the second optimisation step in mm (default: 7)')
 parser.add_argument('-td', '--target_density',
                     help='GIFTI (.shape.gii) file with the target surface density in number of streamlines per vertex '
-                         '(default: uniform per unit of cortical volume)')
+                         '(default: uniform per unit of cortical volume).'
+                         'Can be set to "white", "mid", or "pial" to be uniform on that surface.')
 parser.add_argument('--ignore_bad_dyads', action='store_true',
                     help='ignore alignment constraint for voxels where the dyad is undefined')
 args = parser.parse_args()
@@ -76,6 +77,10 @@ white_surf = CorticalMesh.read(args.white)[surf_mask]
 
 if args.target_density is None:
     target_density = Cortex([white_surf, CorticalMesh.read(args.pial)[surf_mask]]).wedge_volume()
+elif args.target_density in ('white', 'mid', 'pial'):
+    fn = getattr(args, 'mid_thickness' if args.target_density == 'mid' else args.target_density)
+    surf = CorticalMesh.read(fn)[surf_mask]
+    target_density = surf.size_faces()
 else:
     target_density_vertex = nib.load(args.target_density).darrays[0].data[surf_mask]
     target_density = white_surf.graph_connection_point().T.dot(target_density_vertex) / 3.