[Rivet] Check diphoton + X Rivet analysis

[Andy Buckley]

Hi Roman & Kiran,

I've integrated the ATLAS diphoton + X analysis that you supplied into
the Rivet trunk, but there were several opportunities for code tidying
along the way, which I took since it'll make our lives easier
maintenance-wise in the long run.

I don't have easy access to an MC setup that could be used to revalidate
the analysis code, but the changes were not substantial so I thought it
would be fine for you (and the original author) to cast an eye over the
updated code:

https://rivet.hepforge.org/trac/browser/src/Analyses/ATLAS_2012_I1199269.cc

We aim to release Rivet 2.1.1 early next week if possible, so please let
us know soon if you see anything concerning. It is also *just about*
possible that we could squeeze another analysis integration into this
release if you have one which you were just about to send to us...

Best wishes,
Andy & the Rivet team

-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN