Tracing of photons is fun by itself, but usually one is interested in
specific quantities. Therefore it is necessary to register the photon
at each point of interest. Here, absorption is considered which has been
neglected so far: this is done by weighting the photon with a Lambert-Beer
factor that considers the absorption along the path. The absorption optical
thickness is accumulated from scattering point to scattering point,
integrating the absorption coefficient through all model boxes. The process
of tracing fractions of photons is called 'photon weighting'.
