Need to "mask" channels which go outside of some validity range in a combination

[ajgilbert]

Possible way to do it without modifying combine:

• After producing the workspace identify the RooAddPdf for the channel that needs to be masked
• Get a list of its dependent variables
• Replace each variable with a function that evaluates to zero outside the valid range and the original parameter value inside it
• Finally can rebuild the full RooSimultaneous replacing the pdfs that were modified

Now when we go outside the range the channel NLL no longer changes as a function of the parameters, just leaves a constant term which shouldn't affect the minimisation at all.

[ajgilbert]

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.