Allow added cuts
Created by: Zaharid
This PR adds the ability to have additional cut definitions that get added to the internal ones. Internal cuts must be used for this to have any effect. This is useful to make comparisons as a function of some cut, without having to copy and modify all of the rules.
The PR also adds support for zero sized datasets (after cuts), at least enough so we can compute chi2 tables and such. I'd assume other things would crash still.
TODO: Add tests and docs.