bug-fixes to FATRAS photon conversion screening functions
- ISF_FATRAS photon conversion screening functions had two bugs (incorrect signs) in screening function implementation
- The fixes were discussed at the fast chain meeting: https://indico.cern.ch/event/1037188/
- These fixes bring ISF_FATRAS (closer) in line with GEANT4 and ACTS implementations
- Small effect on observable e/gamma distributions ( @tcuhadar : will cause a diff in FastChain nightly test )
Edited by Liza Mijovic