ThOr functor for photon CL

ThOr equivalent for LoKi CL functor (photon CL) is missing.

The implementation for Particle v2 will depend on LHCb#104, but the Particle v1 version can be made already now.

The name CL looks too generic. What should be the best name for it? In line with other PID-related quantities, it should probably be called CL_GAMMA?