Artificial satellite magnitude model

[Bill-Gray]

Find_Orb currently has two magnitude models: a comet mag model, and the standard H/G asteroid model. There is yet another model, commonly used for artificial satellites, in which the magnitude just runs as the fraction of a sphere that would be illuminated at that distance and angle. It's not a great fit -- artsats obviously have a wide range of shapes and their magnitudes can be weird -- but that model is a lot better than the H/G one, which is really intended for rocks.