Make sure Laursen's model is not used

src/assemblers.cc

@@ -90,6 +90,8 @@ assemble_nonlinearity(
           nodalIntegrals, a, mu, eta, normalStress, b, state, L, h);
     }
     case Config::Laursen:
+      assert(false); // Known to be broken
+
       // TODO: take state and h into account
       // FIXME: We should be using a quadratic rather than a linear function
       // here!