[Rivet] Histogram normalisation

[Hendrik Hoeth]

Thus spake Andy Buckley (andy.buckley at ed.ac.uk):

> > And for all others the automatically determined kfactor should
> > somehow be plotted for each MC run in the histogram, maybe together
> > with the legend, or above the top edge?
> 
> Sure. Well, that's a plotting detail, but we can make sure in the YODA
> implementation that KFactor can be written as an annotation and used
> by anyone who wants to.

CustomLegend=<text> adds custom text to the legend in make-plots.

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