dust again

Barron, LeCun, and I complexified our project on modeling galaxy images with three-dimensional galaxy models by considering the case of absorbing dust. We decided that we either have to severely restrict the possible dust geometries or else move to a full ray tracing or computer graphics approach. I prefer the latter, but Barron is (rightly) concerned about speed. The long-term goal of the project is for the computer to simultaneously classify and model all galaxies, and also choose optimal or natural, data-driven parameterizations of the model space.