Correct uncertainty calculation for summary

[swertz]

We need a more robust method for computing the uncertainties in the summary given by plotIt:

MC:
                                                                   N ±         ΔN            ε ±       Δε
(...)
TT_TuneCUETP8M1_13TeV-powheg-pythia8_MiniAODv2_v1…           1626.53 ±       5.54     0.08899% ± 0.00030%
DYJetsToLL_M-10to50_TuneCUETP8M1_13TeV-amcatnloFX…             -9.93 ±       -nan     -0.00002% ± -nan%
DYJetsToLL_M-50_TuneCUETP8M1_13TeV-amcatnloFXFX_M…             20.73 ±       0.02     0.00016% ± 0.00000%
------------------------------------------
Systematic uncertainties
                                        Luminosity                   ±      78.17
------------------------------------------
                                                             1699.29 ±       -nan

I don't really know how to compute an uncertainty on an efficiency when events with negative weights are in the game (if only positive weights, using https://root.cern.ch/doc/master/classTEfficiency.html#ae80c3189bac22b7ad15f57a1476ef75b will be the best choice).

On the other hand, we don't really need this uncertainty, do we? What's interesting is the uncertainty on the predicted yield, which is easier to compute (at the moment this is computed from the uncertainty on the efficiency)...

Any thoughts?