[ENH] G4Radioactivation process is not implementing decay chain time biasing

[BishopWolf]

The old G4RadioactiveDecay process that was implemented in Gate was meant for simple decay chains. However it has some big problems such as not controlling the time biasing of long half lived daughters

The new G4Radioactivation process solves this issue but it must be implemented as in this example. Specially in this file. The relevant lines are:

  // mandatory for G4NuclideTable
  //
  G4NuclideTable::GetInstance()->SetThresholdOfHalfLife(0.1*picosecond);
  G4NuclideTable::GetInstance()->SetLevelTolerance(1.0*eV);

  //read new PhotonEvaporation data set 
  //
  G4DeexPrecoParameters* deex = 
    G4NuclearLevelData::GetInstance()->GetParameters();
  deex->SetCorrelatedGamma(false);
  deex->SetStoreAllLevels(true);
  deex->SetMaxLifeTime(G4NuclideTable::GetInstance()->GetThresholdOfHalfLife()
                /std::log(2.));

As you can see the initial evaporation process defined in G4RadioactiveDecay is not sufficient, it also needs control on the deexitation process to remove very long half lived daughters.

This shall be implemented in Gate to remove some of the emission spectra issues that are mentioned.