[Rivet] implementation of W/Z cross-section ratio analysis

[Roman Lysak]

  Dear Rivet authors,

there is an ATLAS measurement of ratio of W/Z (+1jet) cross-sections, see:

http://inspirehep.net/record/924848

The same  measurement with much more data is also planned.
We were thinking about implementing this analysis into Rivet. Clearly 
this would require some non-standard approach since one would have to 
run over both W and Z sample. However, I can imagine there could be two 
separate routines which people could run on two different samples 
(W,Z).  In the end, the ratio plots would be produced just by dividing 
the individual plots.

Any your comments/suggestions would be appreciated.

Thanks,
   Roman