The subroutine supercell_symmetry searches the supercell
symmetry. When the lattice symmetry decreases, the number of
rotational symmetry operations decreases. All the symmetry operations
in the supercell, which includes pure translations, are used for
following parts and the symmetry operations of unitcell are not used
at all. The rotational symmetry operations which are not those
including pure translations (`symrel_supercell`

) are used in
site_point_symmetry.

togo
2009-02-12