In case a user uses many GeneratePrimaryVertex events in his
PrimaryGeneratorAction and for each PrimaryVertex he uses SetParticleMomentum()
with different momenta every time, then the warning message "Kinetic Energy is
overwritten" is shown for most events after the first one.

This is because the particle_energy left from the first GeneratePrimaryVertex()
(in PrimaryGeneratorAction) is not reset (to zero) after the
GeneratePrimaryVertex (in G4ParticleGun) has finished.

I propose to add a SetInitialValues() at the end of GeneratePrimaryVertex like that

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    particle->SetCharge( particle_charge );
    particle->SetPolarization(particle_polarization.x(),
                               particle_polarization.y(),
                               particle_polarization.z());
    vertex->SetPrimary( particle );
  }


  evt->AddPrimaryVertex( vertex );
  SetInitialValues();


}