Update assign_elm_nodes mapping

As per discussion in #3337

Author: jwpeterson

src/mesh/tetgen_io.C

@@ -206,11 +206,9 @@ void TetGenIO::element_in (std::istream & ele_stream)
   libmesh_error_msg_if(region_attribute > 1,
                        "Invalid region_attribute " << region_attribute << " specified in .ele file.");
 
-  // Vector that assigns element nodes to their correct position.
-  // TetGen is normally 0-based
-  // (right now this is strictly not necessary since it is the identity map,
-  //  but in the future TetGen could change their numbering scheme.)
-  static const unsigned int assign_elm_nodes[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+  // Vector that maps Tetgen node numbering to libMesh node numbering. Tet4s are
+  // numbered identically, but Tet10s are not.
+  static const unsigned int assign_elm_nodes[] = {0, 1, 2, 3, 9, 7, 4, 5, 8, 6};
 
   for (dof_id_type i=0; i<_num_elements; i++)
     {