Implementation of all routines concerning a model photon.
//The object 'photon follows a photon's life from its creation in some
// emission region of the model space, through scattering processes,
// until either its absorption or detection. All necessary routines
// and parameters are defined here, including the procedures for
// electron scattering or scattering off a generalized dust
// grain. Some features are marked with a symbol (*) denoting that
// they are not used by the program at its current state but that they
// are reserved for future purposes or diagnostic reasons.
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