[Rivet] Possible bug and fix in analysis module(s)?

[Jonathan Butterworth]

Hi all,

About 24k events into a rivet run (using the tip of the default branch), 
it terminated with the following message:

Duplicate name 'ZFinder' in parent 'ATLAS_2015_I1408516_MU'

which comes from the projection handler.

I looked into the code, and I think the issue is that the _MU and _EL 
versions of this analysis (it's the ZpT one) use the same projection 
name. I'm running some exotic events which have no Zs, but do have loads 
of muons and occasional electrons. My guess is the problem occurs when 
there is a pair of muons and a pair of electrons in the same event, each 
consistent with a Z - not something you'd get in the SM processes this 
analysis was aimed at, but not something that should crash it.

I have just committed some changes which I think sensibly address the 
problem - certainly they stop the exit mid run and the plots look right 
as far as I can tell.

I noticed at least one other analysis (which I am not using) with the 
same problem. If you agree with my fixes, I will apply them to the other 
routines, but I'd appreciate someone else having a quick look, as I am 
on the edge of my c++ knowledge and there might be a better way.

Also I'm guessing it would be better if projection handler trapped this 
at initialisation, not runtime? But I don't know how to do that (or if 
it's even possible).

Cheers,
Jon

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prof. Jonathan Butterworth,              http://www.hep.ucl.ac.uk/~jmb/
Head, Physics and Astronomy Department            Tel: +44 20 7679 3444
University College London                 Gower St, London WC1E 6BT, UK
ATLAS, CERN                                       Tel: +41 22 76  72340
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