Modified Geant4 example

[gerritzen]

Sorry for the spam.

To get accommodated to NEST, I modified one of the Geant4 examples (see here). I figured this might be useful for others. If you agree, I can either include the code in here and create a PR, or you can include a link in the README. If not, feel free to ignore this issue.

P.S. I didn't find an elegant way to edit a ready-made physics list, so I copy-pasted G4OpticalPhysics and just changed a few lines.