Halved the cost of the pull communication

Halved the cost of the pull communication

With DD the PBC reference coordinates are now only communicated
after DD repartitioning. This reduces the number of MPI_alltoall
calls from 2 to 1 per step, which can significantly improve
performance at high parallelization.

Added a cycle counter for pull potential.

Added checks for zero pull vectors to avoid div by 0.

Change-Id: Ib89ba9e14eaa887f59a5087135580bc29a20d7d0