[BUG]: Urgent fix for data-driven background with MC

The important one is a egression from https://gitlab.cern.ch/cp3-cms/bamboo/-/merge_requests/166: due to a typo the weight would be squared for data-driven contributions. I don't see it in ttW, so I'm a bit puzzled what exactly triggers it (update: understood now: it takes the weights from the "main" selection, but for data there usually isn't any there) I know @fbury has a creative use of the data-driven code, so please check.

To check if you're affected, look for things like this in the verbose logfile:

DEBUG:bamboo.dataframebackend:Plot Melel systematic variation qcdScale0: defining new weight based on w_twoElectrons__qcdScale0                                                                                     
DEBUG:bamboo.dataframebackend:Defining w_twoElectrons2El_Melel__qcdScale0 as ( w_twoElectrons__qcdScale0 * genWeight * myCol1 * LHEScaleWeight[0] * myCol373 * myCol374 )  

(grepping for genWeight in MC is probably a good one, it should be always there, and only enter a few times at the beginning of the log, to define an initial weight and its systematic variations)