L1Calo eFEX Simulation: deal correctly with floating precision rounding effects

deal correctly with floating precision rounding effects in converting energy to et in units of 12.5MeV

This MR is intended as a prereq of the outstanding MR !62722 (merged) which will also see this change implemented. Here I'm breaking out this change to factorise out the effects on the reference files, which I will deal with here.