Adding Exodus node_num_map and elem_num_map stuff back in.

(Reverts a previous revert.)
Adding insert_node() interface at the MeshBase level.

Replacing original (unused) ParallelMesh::insert_node() function
with an implementation that just calls ParallelMesh::add_node().

The mesh_inserter_iterator now uses insert_node() instead of add_node().

These changes are to support broadcasting Meshes with discontiguous
node numberings (NULL pointers in the _nodes vector).