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 G4BertiniElectroNuclearBuilder
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 It includes gamma-nuclear, electron-nuclear and positron-nuclear
 processes.
 For gamma-nuclear, it uses Bertini (BERT) model for gamma below 6 GeV,
 and Quark-Gluon-String (QGS) model above 3 GeV.
 For electron-nuclear and positron-nuclear, it uses the equivalent photon
 approximation in which the incoming lepton generates a virtual photon, 
 and then the virtual photon is converted to a real photon. This real
 photon is handled by BERT if its energy is below 10 GeV; if it is above
 10 GeV, then the real photon is transformed into a (on-shell) pi0 and
 then handled by Fritiof (FTF) string model.

 
 G4EmExtraPhysics
 ----------------
 It uses G4BertiniElectroNuclearBuilder for gamma-nuclear, electron-nuclear
 and positron-nuclear.
 Moreover, it includes muon-nuclear process (for mu- and mu+), and 
 synchrotron process (either for electron & positron, or for all
 charged particles).
 The muon-nuclear process is treated similarly as for electrons and
 positrons (i.e. the equivalent photon approximation in which the incoming
 lepton generates a virtual photon, and then the virtual photon is converted
 to a real photon, which is handled by BERT below 10 GeV or by FTF as a pi0
 above 10 GeV).
 By default, gamma-nuclear, electron-nuclear, positron-nuclear, and
 muon-nuclear are switched on, whereas synchrotron process is switched off
 for all particles. It is however possible, at run time via macro commands,
 to change this default (see G4EmMessenger below).


 G4EmMessenger
 -------------
 Used by G4EmExtraPhysics to be able to switch on/off: 
 -  synchrotron radiation for electron and positron
    (note: electron and positron together, not individually)
 -  synchrotron radiation for all charged particles
    (note: all charged particles together, not individually)
 -  gamma-nuclear, electron-nuclear and positron-nuclear
    (note: all these three particles together, not individually)
 -  muon-nuclear
    (note: mu- and mu+ together, not individually)
 at run time, via macro commands.

