Add a way to validate output histograms against a trusted reference

[eguiraud]

In first approximation it would be enough to publish reference histogram content, in whatever easily readable format (even JSON, or serialized dictionaries of numpy arrays).

A simple CLI tool or similar that runs the comparison between two such JSON files could be provided on top.

Complications:

• one systematic variation depends on RNG numbers, so bin values won't be stable (especially with multi-thread/out-of-order execution): I think it's ok to clearly mark it as such and only expect a very approximate equality there
• bin values depend on how many files per process are used: I think we should provide values for at least 1 file per process (for quick validation) and for the full dataset (for full validation)

[alexander-held]

As different implementations may run into some small discrepancies, I want to link #163 (comment) here for future reference. It can happen that events migrate across bin edges, which (depending on the event weight) can result in a somewhat large absolute change in event yields. I don't think there is anything we can do about this and at the level we observed this is not a concern for physics anyway.