Bring FATRAS conversion child energy sharing code in-line with GEANT4
Bug-fixes to FATRAS photon conversion, discussed and approved at FastChain meeting (https://indico.cern.ch/e/1032355)
- Bring the conversion child energy sharing code in-line with GEANT4: fix energy sharing cross-section function and sampling formulas. The bug-fixes correspond to GEANT4 implementation (and are in-line with ACTS-fatras)
- Set m_childEnergyScaleFactor default to 1: modelling improved after bug-fixes above, SF of 2 no longer required. As SF=1 corresponds to GEANT4 behavior, we choose this as the default.
Test run on 10 single-photon events: /afs/cern.ch/work/l/lmijovic/public/sim/mr