Better determination of magnitude parameters

[Bill-Gray]

At present, when using the H/G magnitude system, H is determined and G is left at 0.15. If the observations cover a large enough range of elongations, and the magnitudes aren't the usual garbage, it would be possible (and desirable) to determine G as well.

This may become more reasonable to do if the new astrometry format, which includes uncertainties, is implemented. At present, trying to determine G from a mix of well-determined and "wild guess" photometry would be pretty much of a fool's errand.

Incidentally, this also applies to some degree with comets: the dependence of magnitude upon distance from the sun is currently fixed, and could be instead a parameter to be determined from a fit to magnitude observations.