Merge pull request #515 from bangerth/terminate-loop-and-error-message

Terminate a loop in a timely manner.

In StraightBoundary::normal_vector(), we currently have a loop that could run
forever. This is not useful. Rather, terminate it after at most 10 iterations
and if it fails output some guidance to what the reason may be.

diff --cc source/grid/tria_boundary.cc
index e296a5f2152f7286435c66b8a8d2a0592a0fe5de,61c0ea8f5e395c3b682735fee0184050ff3159d8..e32916554fcee4103bb6c7941e43a2f0095de74a
--- 1/source/grid/tria_boundary.cc
--- 2/source/grid/tria_boundary.cc
+++ b/source/grid/tria_boundary.cc
@@@ -628,8 -629,16 +629,16 @@@ normal_vector (const typename Triangula
            for (unsigned int k=0; k<spacedim; ++k)
              H[i][j] += grad_F[i][k] * grad_F[j][k];
  
 -      const Point<facedim> delta_xi = -invert(H) * J;
 +      const Tensor<1,facedim> delta_xi = -invert(H) * J;
        xi += delta_xi;
+       ++iteration;
+ 
+       Assert (iteration<10,
+               ExcMessage("The Newton iteration to find the reference point "
+                          "did not converge in 10 iterations. Do you have a "
+                          "deformed cell? (See the glossary for a definition "
+                          "of what a deformed cell is. You may want to output "
+                          "the vertices of your cell."));
  
        if (delta_xi.norm() < eps)
          break;