[Rivet] adding histograms

[Frank Siegert]

Hi David,

> have you kept that up-to-date? David Winn found that he had to modify
> the copy you gave him quite a bit to work with 1.5.0. I don't have any
> other details, though, it may well relate something that he messed up
> originally.

Yes, I have. It was related to a few API changes that Hendrik made in 
lighthisto when he implemented 2D histograms.

Speaking of 2D: At the MC school there was a question of how to plot 2D 
histos. Since we can fill them in Rivet now, and the AIDA output (now 
hopefully) is correct, and make-plots can handle them, I was wondering 
whether anybody has already scripts/patches which allow compare-histos 
to deal with them?

Cheers,
Frank