[Rivet] Method for accessing particles from FastJets projection

[David Bjergaard]

Hi Andy,

> Does Jet::particles() not work for you?
This works great! It will even simplify my code, so its a double win. 
>
> We'll be overhauling the jets and vectors system for the next major
> release, so this is a good time to get requests in! The plan so far is
> for much better FastJet integration to allow the various filtering etc.
> tools to be used directly.
>
Does that mean that Jets will inherit from PseudoJets and then get
dressed with UserInfoBase in a nice way from rivet? 

Also, I would really like the kinematics to be uniform between the two
(most notably Jet::mass(), Jet::mass2() vs. PseudoJet::m(), and
PseudoJet::m()), This prevents code like:
> template <typename lvec> static void dump4vec(lvec four_mom){
>   std::cout<<"( "<<four_mom.pt()<<" [GeV], "<<four_mom.eta()<<", "<<four_mom.phi()<<", "<<four_mom.m()<<" [GeV])"<<std::endl;
> }
from operating on Jets and PseudoJets which defeats the purpose of the
template in the first place. (Of course this is a very minor complaint.)


    Dave