Get symmetry representations of the supercell

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.