Partial implementation of SatSun class.
Similar to SatPoint, but using SolarPosition to calculate geocentric ECI coordinates of the sun rather than SGP4 to model satellite position. Should further internalize solar position calculations and display geometry (like SatPoint). Due to similarities, consider ways to factor a common parent "class" out of SatPoint and SatSun.
Simplified satsun ecf-to-xyz
Removed outdated SolarPosition commets.
SatSun manages its own PointLight
Changed updateSatTrace event to broader updateDisplay;
SatSun responds to updateDisplay events as do SatTrace objects.
Added method comments to SatSun.