Draft: Adapt LbEvtGen to EvtGen member variable renaming [Sim10]

Thomas Lathamrequested to merge
tlatham_evtgen_membervar_rename_sim10 into Sim10

Adapt code in LbEvtGen to upstream changes to member variable names, specifically those in EvtDecayAmp

cc: @gcorti, @kreps, @jback

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