This process handles (e+,e-) pair production by gammas. It
should be used (instead of G4GammaConversion) when low energy details,
such as atomic shell structure, are important.
Note: "low energy" electromagnetic processes such as this should
not be used together with "standard" electromagnetic processes such
as G4eIonisation, or else unpredictable results may occur. The one
exception to this rule is that the standard process
G4MultipleScattering may always be used.