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

[David Grellscheid]

Hi all,

> I can't see an immediate problem with Vector3 Particle::prodPos(). What
> do you think?

How about Particle::origin(). "production position" is such a clumsy 
HepMC-ism. Also, it really is a 4-vector. Not all of the hadrons are 
produced at the same lab time.

The calculation of z0 should certainly not be a member function, but I 
don't think that's what Holger suggested. If you have the collision 
point, a particle's origin, and its momentum, you can work out most of 
these offset calculation tasks (except for the ones that experimentally 
trace back along curved tracks).

   David