Address Thread-checker warnings in `Charginos`, `Gauginos` and `Sleptons` packages
Creation of G4ParticleDefinition objects occurs during the initialization of Geant4 which is done in a single thread.
The syntax used in the Charginos, Gauginos and Sleptons packages follows the syntax used in Geant4:
https://gitlab.cern.ch/atlas-simulation-team/geant4/-/blob/v10.6.3.4/source/particles/leptons/include/G4Electron.hh
https://gitlab.cern.ch/atlas-simulation-team/geant4/-/blob/v10.6.3.4/source/particles/leptons/src/G4Electron.cc
For now we mark the static s_theInstance/theInstance variables as ATLAS_THREAD_SAFE until such time as the
syntax is updated to something neater on the Geant4 side.