new balance command for load-balancing

The issue of poor performance due to load
imbalance in problems with irregular density
distributions has come up from time to time on the mail list.

FYI, there is a new "balance" command (12Feb12 patch)
which can be used to adjust the planar cuts between
processors (in the 3d grid or procs) before or between
runs. In an attempt to balance the number of atoms/proc.


This is effectively a static load-balancing. A dynamic
"fix balance" command will follow (hopefully soon), built
on the same primitives, to adjust things on-the-fly
during a run.

This was motivated by work in Christian Kloss's group
with LIGGGHTS and granular systems where irregular
geometries are quite common.