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
AP_NavEKF : Reduce weighting on GPS when not enough satellites

GPS measurement variance is doubled if only 5 satellites, and quadrupled if 4 or less.
The GPS glitch rejection thresholds remain the same
This will reduce the impact of GPS glitches on attitude.
Copter: allow passthru for ch 9 ~ 14

Based on work by Emile Castelnuovo
AP_NavEKF : Explicitly initialise gpsNoiseScaler to default value
Copter: update AC3.2-rc10 release notes
Copter: version to AC3.2-rc10
Copter: bugfix to condition-yaw for relative angles

Thanks to roque-canales for raising the issue and paradisephil for finding
the specific piece of code that went wrong and suggesting the fix.
AP_NavEKF : Fix bug in reset of position, height and velocity states

If the inertial solution velocity or position needs to be reset to the GPS or baro, the stored state history for the corresponding states should also be reset.
Otherwise the next GPS or baro measurement will be compared to an invalid previous state and will be rejected. This is particularly a problem if IMU saturation or timeout has occurred because the previous states could be out by a large amount
The position state should be reset to a GPS position corrected for velocity and measurement latency. This will make a noticeable difference for high speed flight vehicles, eg 11m at 50m/s.
AP_NavEKF : Fix bug in GPS innovation variance growth calculation

The uncertainty in acceleration is currently only scaled using horizontal accelerations, however during vertical plane aerobatics and high g pullups, misalignment in angles can cause significant horizontal acceleration error.
The scaling now uses the 3D acceleration vector length to better work during vertical plane high g maneouvres.
This error was found during flight testing with 8g pullups