[Rivet] Histogram normalisation

[Hendrik Hoeth]

Thus spake Frank Siegert (frank.siegert at durham.ac.uk):

> I have started to migrate some analyses (in particular the ones where
> I was desperately waiting for the ability to merge runs).
> 
> Once we switch more analyses Professor will have to be adapted to make
> use of the normalisations in *.plot in case there are any.

Already your last commit includes one analysis commonly used by us for
the tuning, so from a Professor point of view the current HEAD version
is not usable for a tuning. The analysis I'm talking about is the
D0_2004 dijet angular distribution paper, which only contains event
shapes.

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