Unofficial fix to Geant4 bug #741

As described in Geant4 bug report #741, optical photons in Geant4 release 7.0 propagate at the phase velocity c/n(E), where E is the energy of the photon. This is a bug because photons in real life propagate at the group velocity vg = c/(n(E)+dn/d(log(E)). In the KamLAND detector, the sum of distance/(v_phase) over all materials between the center of the detector and tube #258 at a certain wavelength is 40.2 ns, while the sum of distance/(v_group) at that wavelength should be 42.3 ns. Here is what we see for photon arrival time without the above patch:

Here is what we see with the patch:

G. Horton-Smith, 2005/04/14