Root level parts

Handles the case when the top-level cell size increases so that
the cells need to be regenerated and particles re-attached.

The technique is to resample the old cell space and use the node
IDs to populate the new one. This should minimise the need for
particle movement, so should be a little less expensive than a 
full initial partition.

If the resample fails we fall back to a full initial partition.
Could be a better idea to use the same technique as in main.c,
but we don't do that.

Fix for issue #31

See merge request !112