Systematic-uncertainty tools

Heppy provides tools to explore and treat the systematic variations of histograms. This notably means combining multiple systematic variations into a single combined systematic. Some examples include:

• Combining the members of a PDF set into the PDF uncertainty
• Adding several uncorrelated uncertainties in quadrature to get a resulting net uncertainty
• Combining a set of bootstrap replicas into a final uncertainty
• Taking the envelope of perturbative QCD scale variations to get an estimate of the fixed-order calculation uncertainty
• etc.

The basic usage is:

1. Create any number of heppy.uncertainty.model instances that specify which variations should be combined and how.
2. Call heppy.uncertainty.combine_copy, taking as arguments a histogram and a list of uncertainty models, to return a copy of the histogram where the combination models have been applied to the variations.