AP_L1_Control

Modified straight line and circle L1 guidance algorithms to enable:

1) Explicit control of tracking loop period and damping which removes previous
   variation in period with speed and fixed damping ratio
2) Explicit control of track capture angle (now set to 45 degrees by default)
3) Removal of restriction on loiter radius being greater than L1 distance

The circle(loiter) control is now a L1 and PD hybrid utilising L1 for waypoint capture and PD control for circle tracking.