Increased constness in {const_,} node_iterator
Increased constness in {const_,} node_iterator

Before, it was possible to change a mesh's Node via a
const_node_iterator, which was a bad idea, and it was possible to change
a mesh's Node* via a node_iterator, which was a horrible idea whose
horror I was only able to fully comprehend after I did it by accident.

dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,libMesh/libmesh