Choose the PBC image of each rotation group atom depending
on the current position of it reference atom. Therefore,
1) in choose_pbc_image() the correctly rotated reference position
has to be used (the addition of the pivot vector was missing)
2) in init_rot_group(), the reference positions have to be
correctly determined for restarts. In these cases, the initial
angle of the reference group is different from zero.

Change-Id: I57f886c507317385619e42bf387e407479b0c08d