Choose the PBC image of each rotation group atom depending

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