Add Dubins' car dynamics derivation exercise in Ch 3 [modeling]

[Richard Murray]

We decided to keep the full steering dynamics model (with center of mass) in Chapter 3 because we need it to generate examples of right half-plane zeros. However, it would be nice to have the simplified (Dubins) dynamics also available. This can be done by setting the parameter a to zero, which gives that $\alpha$ is always zero.

Generate an exercise to work through these details and, if included in the printed text, reference it in the "Vehicle steering" example that appears in Ch 3.

[Richard Murray]

Exercise 3.7 covers this.