[Rivet] Prototype improved cuts system for Rivet

[David Grellscheid]

>
> Classifier
> <
>      And
>         <
>          Classifier<Energy>,
>          Or
>          <
>              Classifier<Eta>,
>              Classifier<Phi>
>          >
>      >
>  >
>

In actual use, you don't see the template mess, but just:

   FinalState( PtGt(500*MeV) && (ModEtaLT(2.5) || NChargedGT(10)) );

The type of the argument here is

   Classifier< And< PtGt, Or< ModEtaLT, NChargedGT > > >

but nobody sees that explicitly, since the declaration of FinalState() is
   template <typename T>
   FinalState( typename Classifier<T> );

See you,

   David