Added pulling quadratic through a linear system. Added unit tests first to validate the math, and then implemented a LinearSystem class and PullThrough(LinearSystem) as a member of QuadraticFunction. Any quadratic function can now be treated as a function over the state of a linear system and pulled back through time one timestep to be the one-step backup quadratic function over states that assumes the policy chooses the best action from there with respect to the original quadratic's cost function.