[Rivet] New LHCB analysis --- definition of z_0

[David Grellscheid]

>> Isn't Beams::IP always going to be (0; 0,0,0)? We used to have a
>> PrimaryVertex projection and it was totally pointless.
>
> I'd still leave it in. ThePEG can generate IP offsets (although nobody
> uses them), and I could imagine other situations that we shouldn't lock
> out.

Let me clarify why for once I'm in favour of a (slightly) more 
complicated option: the fact that all current generators produce 
(0,0,0,0) here is not enforced by any of the tools we use upstream, it's 
just a coincidence. HepMC allows any value in that field, so we should 
be able to handle any value there.

   David