Implement Birks saturation for materials other than liquid argon

[ClarkMcGrew]

A generic Birks calculation should be added for materials that are not Liquid argon. The implementation exists, but it requires setting the ionization properties for a material. I don't know how to do that via GDML. This needs advice on how to attach new material properties to materials defined in GDML.

[ClarkMcGrew]

This is now implemented using the G4EmSaturation() method using the Birks' constant from G4IonisPropertyMat(). There are difficulties setting this value from GDML, so the /edep/material/birksConstant macro has been added as a supplement.