update element weight system
cleanup load balance options

centroid-based diffusion is part of parma now,
and in the long term we want at most two methods
available: a global one and a diffusive one.
The diffusion may be centroid, maybe with other
bits from parma.

Note: this breaks calls in Albany and Proteus !
merge carefully.
reject potential volumes that are exactly zero
Merge branch 'master' into layer