Cell systems should use p_old for sorting

[fweik]

The cell systems are actually sorted by p_old, which should also be used to find the current cell for the particle. This always works, and makes the cell system independent of the geometry, e.g. it can just use normal distance calculation, instead of minimum image convention. For this to work in layered and dd the criterion has to be adapted to use p_old, when adding new particles, p_old has to be set to the position and the particle position rescaling code for NPT has to be adapted to rescale also p_old.