+ Adding support for comet magnitudes.

KStars can now pick up H and G parameters (absolute magnitude and slope parameter) from the comets.dat file if present, and use them to calculate the magnitude of comets.

+ Fixing formula for comet and asteroid magnitudes

The formula used earlier was applicable only to 2P/Encke and was not general, and I had mistakenly put that in.
Now using the generic formula.