Changes

keep_structure: only merge levels which are actually parent/children
We never checked the parent/child relationship because the number of
object in (unrelated) consecutive levels (in depth order) is very often
different. However this is really needed in practice.
Fixes #363
Thanks to Mark Allen for reporting the issue. He had a P9 machine
restricted to only 6 cores. And there were also 6 groups for CPU-less
GPU memory nodes. Those levels are really unrelated, but the core
algorithm decided to place them nearby in the list of depths.
In git master, this issue would only occur if we disable the
auto-ignoring of these GPU nodes.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>