ISF_Geant4Event: const/mutable fixes

Frank Winklmeierrequested to merge
fwinkl/athena:isfg4event into master

Avoid const-violations in Geant4TruthIncident by calculating the list of children already in the constructor instead of on-demand. Hopefully this has negligible impact on performance.

Also use static const initialization in ISFG4GeoHelper and enable thread-checker.

Merge request reports