Added simplified lightFlowPhotonDensity() evaluation routine to get RGB irradiance (_not_ scalar illuminance) from lightflow photons for a given plane normal. This way cubic illuminance can be obtained from 6 successive evaluations and must be calculated in a postprocess, but this approach conforms with RADIANCE's input/output convention and (most importantly) gives access to the raw axis illuminances in RGB tuples if chromatic analyses are anticipated.