Egamma config smarter flags add egamma track slimmer
Following discussion with @tbold make flags a bit smarted add egammaTrackSlimmer alg (in reality we call what it does slimming these days).
This comes after !47485 (merged) and fixes some issues like
File "/tmp/christos/Athena-git/build/x86_64-centos7-gcc8-opt/python/egammaConfig/egammaBuilderConfig.py", line 36, in EGammaReconstructionCfg
flags.Egamma.doTruthAssociation = False
File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc8-opt/2021-10-21T2101/Athena/22.0.46/InstallArea/x86_64-centos7-gcc8-opt/python/AthenaConfiguration/AthConfigFlags.py", line 97, in __setattr__
return self._flags._set( merged, value )
File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc8-opt/2021-10-21T2101/Athena/22.0.46/InstallArea/x86_64-centos7-gcc8-opt/python/AthenaConfiguration/AthConfigFlags.py", line 231, in _set
raise RuntimeError("Attempt to set a flag of an already-locked container")ping @jdevivi