Copter: THR_ACCEL_IMAX param range increased
Copter: reduce throttle to min once landed in RTL

This catches the case where the vehicle lands but the user doesn't
immediately put the throttle to zero.  Before this check it would
continue to attempt to hold it's
Copter: THR_ACCEL_IMAX default to 800
Copter: version to AC3.2-Iris

Interim release 3DRobotics's use with IRIS+ frames
Copter: Log NTUN while in LAND mode with GPS
AP_NavEKF : Reduce sensitivity on filter divergence check

Flying aerobatics with Trad Heli has shown that the divergence check can be false triggered when large magnetometer errors and GPS dropouts are present.
This can also happen with multi rotors if large yaw rates are present.
This was an unintended consequence of the ekfsmoothing patch which improved filter stability during high rate manoeuvres, but made the divergence test more sensitive.
Copter: reset battery_fs after dis/rearming
Copter: define limit for baro vs inav alt disparity
Copter: typo fix for baro vs inav alt disparity definition
AC_WPNav: add loiter_soften_for_landing method

This resets the position target to the current location.
Copter: soften loiter target when maybe landed

Applies to auto's land, land, loiter, pos hold and rtl flight modes
Copter: add land_complete_maybe flag
Copter: rename land_maybe_complete function
Copter: increase Alt Disparity check to 2m
LowPassFilter: add div by zero check
AC_PosControl: 2hz filter on accel error

Replaced hard-coded filter with LowPassFilter class allowing the
filter's to be 2hz on both APM and Pixhawk
AC_PosControl: 4hz filter on z-axis velocity error
Copter: use disparity threshold define for pre-arm checks

There are two duplicate checks, one in the pre-arm checks (i.e. checks
run every 15 seconds or so before the vehicle is armed) and one in the
arming checks (run immediately before arming).  The definition in the
pre-arm checks was still using the old hardcoded value.
Compass: always default devid to zero
AP_AHRS : Prevent EKF starting if GPS sats less than  AHRS_GPS_MINSATS