The new numbering uses the partition information
to sort border nodes first to compactify storages
that need access indexed by border node ID.
We also get an optimized cache performance for free
sincr we can also recursively sort the nodes by cell ID.
This implements issue #3779.
This graph enables efficient boundary edge scans at each level.
Currenly this needs about |V|*|L| bytes of storage.
We can optimize this when the highest boundary nodes ID is << |V|.
This commit removes all occurences of unconnected boundary nodes
and switches to the simple heuristic of picking U for the forward
and V for the backward node. This performs better than several
fancy heuristics.
