[Rivet] yodamerge for Histo1D

[Leif Lönnblad]

I've written down some thoughts on merging Rivet runs:
http://home.thep.lu.se/~leif/misc/weightmerging.pdf

I guess the bottom line is that to do it properly, the merging needs to 
be done before finalizing, but that in certain cases also finalized 
histograms can be handled.

/Leif