Add code and a description on how to calculate the HEC sampling fractions, see ATLASSIM-4453. The HEC sampling fraction code was originally added to 21.3 by @kiryunin and @pavol in !31346 (merged). This is the port to 21.0 (and master). The evgen generation, the G4 simulation and the calibration hit ntuple writing use the same code now as the LAr EM sampling fractions. The HEC sampling fraction analysis code is essentially unchanged from !31346 (merged) apart from minimal adaptations to the different ntuple.