Better support / examples of how to produce histograms weighted according to some variable(s)

[samvanstroud]

Would be very useful when comparing jet distributions with different pT, PU, etc

Currently we either use @nikitapond's resampling code (not sure where that lives) or just place cuts to try and mitigate these effects (see Jackson's slides)

filling histograms with weights would be a better idea :). it would be great to support reweighting in >1 variables. we could just use the uniform distribution as the target.

@zcapjdb @nikitapond @dkobylianskii @afroch