Add/finalise backbone functionality

[leifeld]

Currently part of DNA and rDNA, but needs some work:

• Tim: Check why some weird jumps in the solution occur at certain penalties. Try it multiple times exactly at those penalties.
• Philip: Tweak the algorithm. Perhaps genetic parallelisation and use Boltzmann constant for the cooling schedule to find global optimum without significant computing time loss?
• Both: Application to non-DNA data for paper.
• Anything else?