Fix final snapshot properties

The final dump of stats/snapshot/VR should have a drift but not a drift that initialises the particles. This was triggered by the jenkins suite.

@jborrow somehow, we never caught that the final snapshot had wrong values for things that get initialised in the drift and hence zeroed. Like the densities. Is that really true? Now that the default is SPHENIX, the code is actually crashing on the final dump as we try to gather entropies in the stats and this leads to a division by 0. Did we always have garbage densities in the final snapshots (not with an output list) or does that occur only if we use a fixed number of steps (-n)?