add_single_child Subroutine

  subroutine add_single_child(mg, id, n_boxes_lvl)
    type(mg_t), intent(inout) :: mg
    integer, intent(in)       :: id
    !< Id of box that gets children
    integer, intent(in)       :: n_boxes_lvl
    integer                   :: lvl, c_id

    c_id                     = mg%n_boxes + 1
    mg%n_boxes               = mg%n_boxes + 1
    mg%boxes(id)%children(1) = c_id
    lvl                      = mg%boxes(id)%lvl+1

    mg%boxes(c_id)%rank      = mg%boxes(id)%rank
    mg%boxes(c_id)%ix        = mg%boxes(id)%ix
    mg%boxes(c_id)%lvl       = lvl
    mg%boxes(c_id)%parent    = id
    mg%boxes(c_id)%children  = mg_no_box
    where (mg%boxes(id)%neighbors == mg_physical_boundary)
       mg%boxes(c_id)%neighbors = mg%boxes(id)%neighbors
    elsewhere
       mg%boxes(c_id)%neighbors = mg%boxes(id)%neighbors + n_boxes_lvl
    end where
    mg%boxes(c_id)%r_min = mg%boxes(id)%r_min
    mg%boxes(c_id)%dr(:) = mg%dr(:, lvl)

  end subroutine add_single_child