Attitude.pde : Add logic so that the L1 controller is not used during landing or takeoff.

Don't use the L1 controller for takeoff or landing. At present landing and takeoff control attempts to  maintain
a constant yaw angle which is fundamentally different from the way that the L1 controller works.
The longer term solution is:

a) Takeoff - Send the L1 controller one loiter waypoint which is set a parameter specified distance
from the takeoff point and in the launch direction. The bank angle will then be limited as a function of height above launch height
and the vehicle will fly to that waypoint attempting to climb at the specified pitch angle and throttle.
If it reaches that waypoint and still hasn't achieved the specified height it will loiter whilst attempting to climb.
This will prevent a flyaway for an aircraft that is unable to gain height and provides a more deterministic flightpath
during takeoff and climbout.

b) Landing - rather than hold heading, maintain track using L1 controller, but reduce the bank angle progressively
as a function of estimated height so that by the time flare starts the bank angle is reduced to a parameter specified
value (eg 5 deg)