set_length_chain Subroutine

    subroutine set_length_chain(this, length_x, length_y, length_z)
        class(chain) :: this
        integer, intent(in) :: length_x, length_y
        integer, intent(in), optional :: length_z
        if (present(length_z)) then
            unused_var(length_z)
        end if

        unused_var(length_z)

        ! the input checkin is all done in the calc_nsites routine!
        this%length = this%calc_nsites(length_x, length_y)

    end subroutine set_length_chain