Add Fins Part

[cthunter01]

Add Fins abstract part. It should have a getAeroForces() virtual method, and a getWeightedCM() method that returns the product of mass and CM (center of mass)

Add SquareFins implementation. It should implement getAeroForces() and getWeightedCM() for a 3 Square fins.

Add this to the model::Rocket class so that the propagator takes into account the new methods in determining forces and accelerations.