yoda2yoda: Allow output-file to be split by multiweight
As requested during the Pythia dev meeting: Provides a --split-by-weight
flag to yoda2yoda
that allows splitting of the output file by multiweight.
There's a conceptual issue here, which is that YODA doesn't know anything about weights in we're technically recycling Rivet logic for this. However, given the foreseeable evolution of YODA AnalysisObjects, this seems permissible.