Fix DD exact continuation bug

Fix DD exact continuation bug

With domain decomposition the local atom density, used for setting
the search grid for sorting particles, was based on the local atom
count including atoms/charge groups that would be moved to
neighboring cells. This lead do a different density value, which in turn
could result in a different number of search grid cells and thus
a different summation order during a run versus when continuing a run
from checkpoint, when no atoms would be moved. Now exact continuation
is guaranteed for the domdec module with the mdrun -reprod option.

Refs #2318

Change-Id: I78452c7dfcf3ca6bdee63ece3795efc7e4ac467f