Need to "mask" channels which go outside of some validity range in a combination about combineharvester HOT 1 CLOSED

After some discussion we decided that this wasn't such a great way to go, and instead it will be better, and simpler, to do the masking at the CachingSimNLL level.

A first attempt at this is here: cms-analysis/HiggsAnalysis-CombinedLimit#294

We introduce a RooAbsReal per category, named like mask_CHANNEL that tells the NLL to skip evaluating that category when mask_CHANNEL != 0. By default these are just RooRealVars, created in the new PhysicsModel method getChannelMask(bin), but in our combination physics models we can override this to write formulae that do the masking. Eventually we could also mix and match with masking expressions in the datacards themselves, a bit like how rateParam formulae can be specified, but would need some more work.

from combineharvester.