Celestial body/object trait and module

Created by: ChristopherRabotin

I think this should be its separate submodule so that each object can be in its own file: nyx::bodies::Earth; . This will be quite verbose for sure, but it'll allow for clarity.

Just like in propagators, the root mod.rs should have the trait for the Body. That should include the spherical harmonics of each body, with a possibility of setting those to zero somehow for objects were we don't have that information. It should also be possible to load these harmonics from a file, as is the case in GMAT.

Must include:

• All eight planets
• An SPK file reader with state interpolation method(s?) -- not sure how to fit that in, maybe by having one of the trait methods be the NAIF ID?