Drag modeling at other bodies

GMAT has code that claims to restrict drag modeling to the Earth (see DragForce line 2524), but allows Mars (2523), and throws for any other body. Additionally, the FileManager has code requiring entries for the Earth and Mars, and doesn't appear to provide for other bodies, even if a model is available. The addition of the exponential modeling code was supposed to be generic, but the implementation missed making it so.

This is a blocker for some (outside of Goddard) work.