[Rivet] Particles with PDG ID 0

[Frank Siegert]

Hi all,

I have encountered some Alpgen samples which contain particles of ID 0. 
Rivet throws an exception (from src/Core/ParticleName.cc) because of 
that. Now of course it doesn't make much sense to have a particle of ID 
0, but should we maybe be a bit more robust against something like this 
and only print a warning or do you think it's ok to throw an exception?

Cheers,
Frank