central pixel "cut off" for the pressure profile

[samueldmcdermott]

there is an inherent ambiguity in the value of the pressure profile for the central pixel, since the analytic fit to the Battaglia profile diverges to infinity at zero distance. I made a keyword argument to facilitate checks of the cutoff of the central pixel in this branch. My results indicated that changing this cutoff value had only very small impacts on the central pixel brightness -- eg O(10%) changes in brightness when changing ctr_pix_res as defined in L291 over several orders of magnitude. This indicates a logarithmic divergence, which seems plausible.