ME-only weights not suppressed by default (?)
Looks like the ME-only-type weight variations are always written out on master, by default with cross-section set to -1.0 leading to awkward plots like this:
Adding HEPMC_INCLUDE_ME_ONLY_VARIATIONS: 1
restores the right correct cross-sections, but we probably should just not write them out to begin with, unless that tag is enabled.