weight-handling tweaks and Py3 fixes
This is following up from the discussion in !96 (merged).
- I've fixed a few Py3 issues for
rivet-merge
, would be good if someone with Py2 could check that it still works for them. - Tweaked
MC_XS
andMC_WEIGHTS
slightly to use less calls to thehandler()
- added a proposed solution for the potential
EventMixingFinalState
issue - @bierlich, please review this part when you get a chance and let me know what you think.