Replace LinearSystemId factories with nonlinear models

[calcmogul]

With the extra compute power of SystemCore, higher fidelity models are an option.

Instead of providing linear model factories with the nonlinear parts in a sim physics class, we could provide nonlinear models for each system of the form dx/dt = f(x, u).

An LTVController class would linearize those functions for LQR. Numerical integrators can be thrown at them too, though wrapper classes for ergonomic handles may still be desired.