Plane: added TKOFF_THR_MAX parameter
AP_NavEKF: Increase divergence test margin based on analysis of more user flight logs

Analysis of copter logs has shown cases with a healthy EKF where spikes in EKF4.DS of up to 25% of the threshold have occurred.
A value of closer to 10% for normal operation is preferred.
AP_Compass: added COMPASS_PRIMARY parameter

this allows selection of which compass is the primary. Useful if the
first compass starts giving spurious data (as happened in our plane)
HAL_PX4: fixed use of FMU servo pins as digital inputs

these pins can be PWM output or digital input or digital output
Copter: remove command_description.txt

Description in this file were out of date or incomplete and it is better
for people to refer to the MAVLink documentation or the
/libraries/GCS_MAVLink/message_definitions/common.xml and
ardupilotmega.xml files
AutoTest: add AVC_copter to location

The existing AVC location is for Rovers.  Planes and copters take-off
from a position slighly further north.
Copter: enable PARACHUTE and RALLY for all but APM
Copter: AC3.2-rc2 release notes and version
Copter: remove unused pos vector functions

removed pv_latlon_to_vector, pv_get_lat, pv_get_lon.
it would probably be best if these were moved into the Math library
GCS_MAVLink: moved 3 more send_*() functions to GCS_Common.cpp
Rover: use new GCS_Common.cpp functions
Copter: use new GCS_Common.cpp functions
Plane: use new GCS_Common.cpp functions
AntennaTracker: use new GCS_Common.cpp functions
Copter: release notes for AC3.1.5
AC_PID: Add new AC_HELI_PID as a child of AC_PID
AC_PID: Change Private members to Protected so that AC_HELI_PID can access them.
AC_PID: update example sketch to test AC_HELI_PID
Copter: Change TradHeli to use new AC_HELI_PID class.
AC_AttitudeControl_Heli: Change to use AC_HELI_PID class instead of AC_PID.  Remove FF parameters from class.