Resolve "possible division by zero in DynamicDiffusive load balancer"

If a block has no outflow division by zero may occur here: https://i10git.cs.fau.de/walberla/walberla/blob/master/src/blockforest/loadbalancing/DynamicDiffusive.h#L344

This can easily be reproduced in a perfectly balanced simulation and load balancing with global information enabled and early abort disabled.

Closes #41 (closed)