Dynamics module

Created by: ChristopherRabotin

There should be a module which has most of the dynamics included, such as two_body_dynamics. Therefore, when doing two body dynamics with spherical harmonics, a developer should be able to call two_body_dynamics and harmonics on two parallel threads (using a clone of the state) for these to be applied strictly at the same position. This also allows the dynamics to be identical regardless of the calling order.

I'm not sure how to organize this: a subsequent issue will include attitude computations (and control algorithms) which may be included in the dynamics module. But I'm concerned that might be confusing. So, I gotta figure that out.

Must include:

• Two body dynamics
• Spherical harmonics
• Third body dynamics (which can be called several times with different bodies, e.g. Jupiter on one thread, the Moon on the other)
• Drag models from GMAT (check literature to only support the best representations)
• Solar Radiation Pressure models (at least Cannonball, but more is better)