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