Photon process

Hi @apricePhy,

I added PhotonProduction to the examples for Sherpa and gave Babayaga a try (runs awkwardly).

One thing I noticed:
- Babayaga wants thetaMin and thetaMax in degrees
- I added a conversion to the Selector class so that we can transform theta to eta (useful for the sherpa selector at least)
- we should probably agree on which unit we want for the angles? (degree or rad)
- the executable (called babayaga_fcc.exe) refuses all sqrts greater the 91.0GeV :(

Babayaga provides an lhe file, but the line for cross sections is empty, I will check with Fulvio if he knows,.....

There is a problem in the definition of the finalstate for whizard.py:
- the finalstate is generated from get_final_pdg_lis()
- which is list(_finfo)
- _finfo is filled in a loop as _finfo[p] = particle.,....
This usually works, but if we have two equal particles in the final state (gamma, gamma) or ZZ, the list(_finfo) will return only a single particle.
Need to look at this....

Start with the PhotonProduction examples for Sherpa, the Selector class, babayaga_fcc.exe, and whizard.py; reproduce the reported angle, sqrt, cross-section, and duplicate-final-state observations. Read get_final_pdg_list() and the _finfo population loop, then confirm the agreed angle unit and define done as resolving or clearly documenting each reported problem.

Written by the indexing model from the issue text.