[Rivet] Suggestion: change to compare-histos default behaviour

[Frank Siegert]

Hi Andy,

I'm not worried about backwards compatibility in this case, but I am 
worried about analyses where the refdata has a lot of histograms that 
aren't filled by the MC. In those cases, with the --all option by 
default, they will be plotted without any MC lines in them. Just think 
of correlation matrices in some of the aida files. I've been discussing 
this with David and Hendrik the other day, and we thought the neatest 
solution would be to (by default?) plot histos if they contain only one 
MC line, but not if they contain only the data line.
I think Hendrik wanted to implement this within compare-histos, I have 
followed in the last days though whether he has yet.

Frank