More Models!

Added a Cartesian Model, restructured to allow easy configuration through vector<string>-based state component enabling.

Generalized EKF

Library supports creating any type of model and sensor that fits the bounds of the 18-state defined in include/estimator.h. Sensors and Models both have the ability to update other models with their output estimates.

Ackermann EKF Model

Added a new standard model: AckermannFilter. This models a basic Ackermann/Bicycle system for the EKF update step.