A few fixes and improvements, mainly for data-driven backgrounds

Small fixes:

• the error message if there is an empty list of input files (KeyError before)
• pass plotopts of the first contribution to DerivedPlot (e.g SummedPlot) by default, such that the style is consistent

Data-driven backgrounds:

• fix DerivedPlot (SummedPlot was adding up the nominal contributions instead of the data-driven backgrounds)
• save yields.tex in the output directory (instead of current - this was an inconsistency with non-datadriven)
• split the logic of DataDrivenBackgroundHistogramsModule.postprocess into a few parts, to reduce duplication when implementing a custom postprocess method

Also added: data-driven backgrounds in cutflowreport printout and yields tables