Organize SystemBodies classes into appropriately named modules

[AdamAngle]

Right now, the SunBody and PlanetBody classes are located inside the SystemBodies module. However, there is also an unused module named OrbitalBody that could be deleted. The PlanetBody could actually inherit certain things from the OrbitalBody class (since it orbits), then have the Sun and Planets be abstracted away into only containing their specific JPL Horizons values and rendering routines, which allows for other types of objects, like parabolic orbits (for comets), to be created.