Resolve "Revisit architecture for pure pursuit"

I would like to propose some changes to the interfaces for controllers based on some late feedback in !62 (comment 167282556)

In particular:

• An assumption that single track models are appropriate for the vehicle platform
• Support for vehicle dynamics models (i.e. adding lateral vehicle velocity)
• Representational preference for vehicle dynamics models as opposed to vehicle kinematics models, i.e.:
  • Wheel angle instead of curvature
  • Vehicle frame is assumed to be center of gravity instead of center of rear axle
• Control output are assumed to be reference

I have also explored some uses cases in a design document which should hopefully motivate these changes and the general representation of the given interfaces.

Closes #62 (closed)