Make the result of `dt_geometric_factors` in 1D consistent with other dimensions (#225)

* return a radius, not a diameter

* add comment

grudge/dt_utils.py

@@ -266,7 +266,8 @@ def dt_geometric_factors(
     )
 
     if dcoll.dim == 1:
-        return freeze(cell_vols)
+        # Inscribed "circle" radius is half the cell size
+        return freeze(cell_vols/2)
 
     dd_face = DOFDesc("all_faces", dd.discretization_tag)
     face_discr = dcoll.discr_from_dd(dd_face)