[Rivet] Proposal for multi-weight/NLO counter-event support, and re-entry to the event loop / finalize steps

[Frank Siegert]

>> Actually, we can do simple run merging at the YODA level now, thanks to
>> Dave M putting together the remaining Python += operators for YODA
>> histograms, and a yodamerge script which uses them to combine multiple
>> runs into one. It's very simple at the moment, but we will add a command
>> line way to specify weights for each input run: will that be enough to
>> combine *different* processes by cross-section? Please try it out... it
>> *seems* to be working nicely, given a simple test.
>
> Cool, I'll be trying this out right away.

I have tested this now using three runs with 500, 1000 and 2000
events, but it doesn't seem to be working for me:

http://fsiegert.web.cern.ch/fsiegert/tmp/yodatest/plots/MC_ZINC/index.html

I analysed, merged and plotted the three samples using

 $ rivet -a MC_ZINC -H Rivet.${nevt}.yoda ${nevt}.hepmc
 $ yodamerge -o Rivet.yoda Rivet.*.yoda
 $ rivet-mkhtml Rivet.yoda Rivet.*.yoda

Does that sound correct? Now this is my first test after switching to
hg and it's completely possible that I screwed something up. This is
with the hg repo from two days ago (I have yet to understand the full
update workflow in hg ;-))

I've uploaded the event files to
http://fsiegert.web.cern.ch/fsiegert/tmp/yodatest/{500,1000,2000}.hepmc
if somebody wants to give it a try.

Cheers,
Frank