[Rivet] Bare leptons in MC_Z routines

[Gavin Hesketh]

Hi,
It would be really useful to add more plots to the MC_ZINC, MC_ZJETS 
(and could do the same for the W) using bare leptons (default is 
dressed). We just found out the hard way that this would have revealed a 
problem with some ATLAS MC generation much earlier...

Any objections to doing this? I can make these changes, but would also 
be happy if someone else volunteered to do it also :)

cheers,
Gavin