AP_L1_Control: added lateral acceleration interface

this will be used by the Rover code for steering
Rover: use L1 controller for navigation

this uses the AP_L1_Control library for rover navigation
AP_L1_Control: use a better ground speed estimate at low speeds

this should prevent a rover going in the wrong direction at the start
of a mission
Rover: use a default max turn g force of 2g
AP_L1_Control: use projected position for turns
fixup groundspeed