Fix magnitude model parameter names for MPC comets
Fix magnitude model parameter names for MPC comets

When cutting and pasting the minor-planet code to build the MPC comet
dataframe, I failed to double-check which magnitude model they use for
comets.  Though they themselves fail to document which parameters are
intended, their XEphem data file confirms that these are g/k not HG.

Should I throw back in a pair of aliases for the old names in case they
are already in widespread use?  I’m not sure how to cleanly accomplish
that, and the names were never documented.  I wonder how many people are
already using them under these wrong names, aside from the person
reporting this issue?  In any case, fixes #416.

skyfielders/python-skyfield,skyfielders/python-skyfield