G4ExternalDecay: thread-checker fixes

Cleanup the Pythia8ForDecays singleton (no need for complicated mutex logic, static initialization is thread-safe) and mark is as not thread-safe because it exposes a non-const pointer to a static.

assigned to @fwinkl

This merge request affects 1 package:

• Simulation/G4Extensions/G4ExternalDecay

This merge request affects 4 files:

• Simulation/G4Extensions/G4ExternalDecay/CMakeLists.txt
• Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/ATLAS_CHECK_THREAD_SAFETY
• Simulation/G4Extensions/G4ExternalDecay/G4ExternalDecay/Pythia8ForDecays.h
• Simulation/G4Extensions/G4ExternalDecay/src/Pythia8ForDecays.cxx

Adding @jchapman ,@zmarshal as watchers

added Simulation master review-pending-level-1 labels

CI Result SUCCESS (hash 9f11f51a)

	Athena	AthSimulation	AthGeneration	AnalysisBase	AthAnalysis	DetCommon
externals
cmake
make
tests

Full details available on this CI monitor view. Check the JIRA CI status board for known problems
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthAnalysis: number of compilation errors 0, warnings 0
DetCommon: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 57969]

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit f6fee5d6

added sweep:ignore label