[Rivet] HepMC multi weight convention for central weight

[Holger Schulz]

Hi all,

the alpha version of rivet 3 (which supports multiple weights natively)
triggered a discussion on what the central weight name should be.

As far as I know there is only a convention for scale and pdf variations 
in the
2013 LesHouches proceedings.
What was (I think) agreed upon is that the central weight should simply come
first in the weight vector when writing out to HepMc. This however
states a tiny technical problem as the write out of the WeightContainer
in HepMc2 does not preserve the order of weights.

Within Sherpa the name "Weight" was used and if I understand correctly,
would also be fine from Stefan Prestel's point of view.

It is maybe not that urgent but it would help us in the development
of HepMC and Rivet 3 if we could agree on a name for the central
weight.


Thanks,
Holger