yet more derivatives

I spent the day working out derivatives of my diffraction imaging model with respect to parameters. The crazy thing is that there is a sum over components in the density model (for the molecule) and then a sum over photons within the image (or instance) and then a sum over orientation-angle samples, and finally a sum over images (or instances). And various of these sums have logs and exps inside them. So it is a mess! I wrote them out with a pen on paper, and then typed them up in a nascent paper. The short-term plan is to get to stochastic gradient.