[Rivet] Cuts rewrite syntax feedback

[David Grellscheid]

> It's good, but a bit more verbose than I had had in mind.

Yes, that's been my worry, too. One thing would be to avoid the Cuts
namespace by importing the symbols on an analysis-by-analysis basis?

> Would it be possible to make cosmetic wrapper functions for the most
> common ones, e.g. EtaIn(-2,2) & PtGtr(5*GeV)

No problem, but which ones? The two you listed are by far the most
common, but now we've got exactly the "Gtr-Less" crud we wanted to avoid
in the design in the first place.

There is a need for an is-in-range expression, but the other one can
just as well be (pT > 5*GeV) assuming we import the Cuts namespace.

Do we need other shortcuts, too?

> Just one cosmetic/consistency point: through the Rivet API we've got 
> "pT" rather than "pt" for transverse momentum, so I think the
> Cuts::pt should follow the same convention. Mea culpa for making it
> that way in the first place, but since it's the convention we should
> stick to it.

I'll fix it.

  David