Dynamically load celestial bodies inside CelestialBodies.cs

Load bodies and their statistics from the live planetary tree.  This allows compatibility of the VAB and SPH statistics with future planets and planetary addition mods.  Bryce Schroeder and I are working on a replacement for Planet Factory called Kopernicus.  One person alpha testing our mod noticed that custom planets didn't show up in Kerbal Engineer's VAB and SPH view.  I've modified CelestialBodies.cs to load its content from the live tree, and BuildEngineer.cs to cope with a potentially unavailable planetary tree, since our mod rewrites the system prefab before the planets spawn, but after part manager starts.