[Rivet] Rivet 1.x update

[Andy Buckley]

Hi all,

Despite having said that we were ready to release Rivet 1.8.4, I decided
that it will probably smooth the transition to 2.x if we provide some
behaviour changes in the AIDA version to match YODA version behaviours.
The main one of these is the change to allow the W/ZFinders to exclude
hadronic decay photons (as experimentally calibrated) from dressed
leptons definitions.

I made this change on both the 1.x and 2.x branches over the last couple
of days, along with some math utils / ParticleBase extensions and
further W/ZFinder constructor changes for the 2.x branch, with the
hadronic photons now excluded by default. It'll need a validation run
through mcplots before releasing the final 1.x version, but I'm glad
that this feature addition is now in there -- it will reduce
complaining/demanding another 1.x, and ease the migration work for
analyses that the experiments are currently writing based on the AIDA
version.

However, since there are API and behavioural changes this can;t be the
planned version 1.8.4, so I suggest we call this one Rivet 1.9.0: okay?

I'll send this for testing by Genser/mcplots tomorrow, but will hold
back on releasing until we have an ATLAS decision about this new ttbar
analysis.

Andy

PS. I worked out how to enable commit/push emails from the hg server. If
you want to get these, you can edit the .hg/hgrc file in the
public/rivet repo directory on the website: the list of email addresses
is quite obvious. I think maybe better that people explicitly subscribe
themselves for now rather than sending them to a mailing list.

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