Copter: Leonard's fix for get_loiter_accel_to_lean_angle
Copter: reduce throttle rate D to zero
Plane: set loiter.direction for GUIDED mode
AP_Scheduler: added SCHED_DEBUG=2

used to show overruns and slips
Copter: fixed format of PM message log
Copter: remove unused throttle functions

removed get_pilot_desired_acceleration vand get_pilot_desired_direct_alt
were used for initial testing of the inertial alt-hold controller but
are no longer required.
HAL_AVR: constrain Vcc used in voltage_average()

this ensures a bad Vcc reading doesn't throw off analog inputs too
much
AC_WPNav: optimise to reduce memory copies

save a few cycles

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
AP_Buffer: instantiate as a .cpp file, and optimise for const usage
AP_InertialNav: reduce number of multiplies
AP_InertialNav: make some more functions const
AP_Param: make things a bit more efficient
AP_InertialNav: optimise some multiplies
Compass: use const references for some functions
AP_Buffer: remove bool return from add() method
AP_AHRS: use const references where possible
Copter: use the right constrain() type in Attitude code

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Copter: use a reference to DCM matrix
AP_Motors: use the right constrain() function
Copter: more constrain fixes