Fixed memory leaks, added UNTESTED code to handle single contribution bin (so basically no binning) in preComputeContrib(). Precomputed per-modifier contrib pmaps are now stored in a LUT within the main/parent contrib photon map, which basically just acts as a container. Added skeleton saveContribPmap() as hook for savePhotonMap() to save the constituent precomputed contrib photon maps in the parent pmap's LUT. This code needs elaboration, and savePhotonMaps() needs to be aware of it.