resolve disulfid bond network

So far, disulfids have been detected using a scoring scheme. If
two cysteines fulfill a certain threshold, they're assumed to build
a disulfide bond. This worked with the first come first served
principle. Once bound, it cannot participate in any other disulfid
bond.

The new approach first detects all possible disulfid bonds and
then finds the mutually exclusive set of disulfid bonds optimizing
the disulfid score.