AC : Replacement for Circle mode

Keeps existing functionality, but uses vector fields instead of chaining the rabbit.
Adds a define called #define CIRCLE_GRADIENT 400.0
This is the cm gradient to calculate the strength of the vector field. Could be adjusted for tighter hold in inertial nav.
wp_bearing is altered in this approach. In Arduplane we have two variables, target_bearing and nav_bearing. We may need to go back to this approach since it maintains the actual WP bearing and the angle we're being asked to fly which may be different.

Tested this in the Flash Sim and works well. Even better with Inertial nav enabled.