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.

[mszydagis]

@KianTrengove Can you take a look at this issue?

[KianTrengove]

It looks good to me - it ran on my computer without issue. @mszydagis I'm not sure if we want to have an upper limit on the size of the examples? Most of the ones currently in the example folder are just 1 file, so maybe instead of including a whole Geant4 example we could just have LXeOpticalPhysics.cc and LXeOpticalPhysics.hh there and include a link in either the readme or within those files if it's unclear to people how to use them.

[mszydagis]

Yes, that sounds reasonable to me, @KianTrengove. Could you please work with @gerritzen on getting this done and into NEST? (e.g., a PR for me to approve). And, apologies, @gerritzen for leaving you hanging for over 2 years on this issue (!)

[KianTrengove]

Sure, @gerritzen you can email me at ktrengove@albany.edu if you have questions. Also I hadn't even realized this issue was made 2 years ago, haha.