Fix bug local face numbering for hexes.

Modify routine face_local_num_int for FAMILY_CUBE elements so it uses the local node numbers rather than the vertex indices. When
setting up the face_lno in add_faces, the functions are passed the vertex indices for the face rather than the vertex node numbers.

Previously the local face node numbering was incorrect for hex elements, where for some elements (not all) face_local_nodes
could return the same local nodes for two different faces. This could and most probably will cause any calculation using hexes and
face information (e.g. DG) to fail.

Added unit test for the function face_local_num for linear hex.