Add suppot for crosslinking mass spectrometry experiments

[wfondrie]

In crosslinking mass spectrometry experiments, we try to detect two peptides that are covalently linked from a single MS2 spectrum. Because the pairs can take various forms (target-target, target-decoy, decoy-decoy), a different approach to estimating FDR is needed.

This PR implements the approach from Walzthoeni et al by adding the CrosslinkPsmDataset and CrosslinkConfidence classes to mokapot!

There's still a bit to do:

• [ ] Finish building out the unit and system tests
• [ ] Update the documentation