Draft: Peakfinding, soft veto, and OnDA / CrystFEL adapter

This is not really intended for merging in anything like its current state. Just opening this MR to make the different parts involved easily assessable.

The soft veto feature (based on Philipp's sketch) is of more general interest. If there's a use-case for it, we can cherry-pick the required modifications to the ShmemTrainMatcher and something like the PeakMatcher or the SoftVetoArbiter (depending on how general and metro-like the result should be) into their own branch for field testing.

At the time of writing, CrystFEL does not complain about the output from the adapter, but I haven't gotten it to index anything (and it is a bit of work to set up test).