This method returns the sigma value of the gb.
        If using 'quick_gen' to generate GB, this value is not valid.

def sigma(self):
        """
        This method returns the sigma value of the gb.
        If using 'quick_gen' to generate GB, this value is not valid.
        """
        return int(round(self.oriented_unit_cell.volume / self.init_cell.volume))