More examples, combine support, BDT/MVA support

[jeyserma]

• Added support for BDT/MVA using XGBoost: helper class in Python and C++ added to addons directory
• Combine support: create Combined datacards from the final stage/histmaker outputs
• More examples: MVA, Higgs mass/xsec, jet clustering

Will write documentation for all these additions. We could add these examples to the CI for continuous testing.

Other changes:

• Added meta info for the final step (similar to histmaker) - xsec, lumi, etc. + fixed optional lumi scaling in the final step
• wildcard processes for the final step: in case processList is not defined or empty, it looks for all the processes in the input directory
• RunRDF - change the number of processed events from a DF count rather than sum over events in all files (eventually could remove the count of the number of events in each file - can be time-consuming)

[kjvbrt]

Nice :) Can you add a list of commands how to run the whole chain to each of the three examples?