[Rivet] Rivet validation of higgs- > gamma/gamma plugin

[Holger Schulz]

Hi Michaela,

in preparation of tomorrows release of Rivet we are currently validating 
the contrib plugins
and I am having trouble with yours.

Maybe you could tell me your validation procedure, i.e. which MC you ran 
with what channels
and so forth.

I am seeing a factor of 10 discrapancy in a sherpa generic H->gamma 
gamma setup.

Best,
Holger