Fix Segfault with backrub. (#758)
While BackrubProtocol clears the BackrubMover's segments between
proteins, the bond_angle_map_ data set by add_segment() is not.
This means that when you apply backrub to a large protein and then to a
small protein, the residue numbers from the large protein cause a
segmentation fault in optimize_branch_angles() when the residual data in
bond_angle_map_ is applied to the small protein.
Clearing bond_angle_map_ when we clear the segments addresses the issue.