fix draw_cuboid

gridlock/grid.py

@@ -586,7 +586,7 @@ class Grid(object):
         p = numpy.array([[-dimensions[0], +dimensions[1]],
                          [+dimensions[0], +dimensions[1]],
                          [+dimensions[0], -dimensions[1]],
-                         [-dimensions[0], -dimensions[1]]], dtype=float)
+                         [-dimensions[0], -dimensions[1]]], dtype=float) / 2
         thickness = dimensions[2]
         self.draw_polygon(Direction.z, center, p, thickness, eps)