advance Subroutine

  subroutine advance (istep)
    use fields_implicit, only: advance_implicit
    use fields_test, only: advance_test
    use fields_local, only: advance_local
    use fields_gf_local, only: advance_gf_local

    implicit none
    integer, intent (in) :: istep

    select case (fieldopt_switch)
    case (fieldopt_implicit)
       call advance_implicit (istep, remove_zonal_flows_switch)
    case (fieldopt_test)
       call advance_test (istep)
    case (fieldopt_local)
       call advance_local (istep, remove_zonal_flows_switch)
    case (fieldopt_gf_local)
       call advance_gf_local (istep, remove_zonal_flows_switch)
    end select
  end subroutine advance