FixedWingFollower: Allow circling in PH/RTH

Previously only a position to fly to was allow in the
fixed wing follower which resulted in repeatedly passing
over that coordinate and turning around.
Paths: Make ModeParameter the radius for circle position

Previously the radius was calculated from the start mine end
position but there is no reason to use that representation
other than to be confusing.
RTH/PH: Mode is now airframe type dependent

Fixed wing aircrafts should hold position by circling
a location.  Hardcoding to circle left with 50 m radius.

VTOL behavior unchanged and tries to hold exact location.
FixedWingFollower: Use math library bound
FixedWingFollower: Only update objects that changed

Previously every object was updated every cycle.  This reduces
the amount of locking of the semaphore.
FixedWingFollower: Remove some unused object accesses
Simulation: Add airspeed to fixed wing simulation
FixedWing: Some style clean up
FixedWingFollower: Make RTH/PH radius settable
FW Sim: Add comment about assumptions
Merge pull request #392 from peabody124/fw_rth

Fix RTH on FW