mg_number_of_unknowns Function

  function mg_number_of_unknowns(mg) result(n_unknowns)
    type(mg_t), intent(in) :: mg
    integer                :: lvl
    integer(i8)            :: n_unknowns

    n_unknowns = 0
    do lvl = mg%first_normal_lvl, mg%highest_lvl
       n_unknowns = n_unknowns + size(mg%lvls(lvl)%leaves)
    end do
    n_unknowns = n_unknowns * int(mg%box_size**3, i8)
  end function mg_number_of_unknowns