calc_nsites_chain Function

    function calc_nsites_chain(this, length_x, length_y, length_z) result(n_sites)
        ! i acually do not want to rely on the previous calculated
        ! length object in the class, since it is easy to recalc from
        ! here and so i remove some dependencies..
        ! nah.. i can reuse this here in the set_length routine!
        ! since the length equal the number of sites in the chain!
        class(chain) :: this
        integer, intent(in) :: length_x, length_y
        integer, intent(in), optional :: length_z
        integer :: n_sites
        character(*), parameter :: this_routine = "calc_nsites_chain"

        unused_var(this)
        if (present(length_z)) then
            unused_var(length_z)
        end if

        if (max(length_x, length_y) < 1 .or. min(length_x, length_y) > 1 .or. &
            min(length_x, length_y) < 0) then
            n_sites = -1
            call stop_all(this_routine, "something went wrong in length input!")

        else
            n_sites = max(length_x, length_y)

        end if

    end function calc_nsites_chain