Fixed calculation of slab buffer size for flexible potentials.

The slab buffer size is difference in slabs between the reference and
the current positions of the rotation group. This was originally
calculated once in the init_rot_group() routine. However, it needs
to be calculated at every step since it changes when the rotation group
expands or contracts along the rotation vector.

Change-Id: I3b1fac41f88744e14e712454ae546c94f9b26750