Investigate post-fit sampling for the cases with high statistics and large lnN uncertainties

[anigamova]

Post-fit uncertainty bands are not correctly evaluated from the cov. matrix sampling for bins with high statistics and large lnN uncertainties, reported at CMSTalk.

[kcormi]

This same issue appears for asymmetric gaussian uncertainties with high statistics as well.

As suggested by others, one thing we may want to look into is using the 68% quantile of the sampled toys rather than the variance directly in the computation. I think there may be some other underlying issues in this case that might not be solved, but in general using the asymmetric interval might be a useful change.

[ajgilbert]

See nice summary from @nsmith- here: https://cms-talk.web.cern.ch/t/postfit-uncertainty-bands-very-large/20967/25
Though it doesn't fix the problem, we should align the calculation in CH with the one in FitDiagnostics (subtract from mean instead of best-fit values).

[anigamova]

Thanks @ajgilbert
Would this work #298 ?