Updated code to match Software Design Document better

GameObject objects are now Entity objects. Entity objects are now Nodes.

Entity objects are now attached to parent Node objects. Parent Node objects are responsible for applying gravity to their children and calling their update methods.

Removed the Player class and the HUD code. Additional work will be needed to add a player into the world (namely preventing it from falling through the surface of the planet). Was trying to simplify and clean up code, so I removed the HUD code as I don't think it was great code and not sure if people were using it.

Left Sun in as a distinct object so that the flare and colour will be set automatically.