Handles cyclic group molecules.

def _proc_cyclic(self):
        """
        Handles cyclic group molecules.
        """
        main_axis, rot = max(self.rot_sym, key=lambda v: v[1])
        self.sch_symbol = "C{}".format(rot)
        mirror_type = self._find_mirror(main_axis)
        if mirror_type == "h":
            self.sch_symbol += "h"
        elif mirror_type == "v":
            self.sch_symbol += "v"
        elif mirror_type == "":
            if self.is_valid_op(SymmOp.rotoreflection(main_axis,
                                                      angle=180 / rot)):
                self.sch_symbol = "S{}".format(2 * rot)