autotest: update rover sim for turning circle based steering

more realistic modelling of a rover
autotest: make rover sim closer to Toms slash
Rover: fixed change of speed command

m/s not cm/s
Rover: use L1 control for navigation

this uses the AP_L1_Controller for navigation
L1_Control: expose lateral acceleration demand

this will allow the rover code to directly use the demanded
acceleration
Rover: use acceleration demand from L1

this adds TURN_CIRCLE and TURN_MAX_G parameters, and uses them to
generate a steering angle based on the real properties of the rover
and the L1 lateral acceleration
Rover: improved low speed steering

don't turn too fast
Rover: fixed obstacle avoidance with L1