this prevents an initialisation error, and is reasonable in flight too

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>

we need to check health of each accelerometer separately

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>

will be used to indicate AHRS health

Divergence is now detected by looking for very large changes in the gyro
bias. This will cause the filter to be reset and declared unhealthy for
10 seconds.

Don't reset filterDiverged status immediately during reset

Set filterDiverged true if covariance blows up

Add fault status reporting

this will be used to report when the AHRS subsystem becomes unhealthy

logs all accel data at high rate to SD card

This makes it consistent with rover and copter and clarifies where the
pin is on the Pixhawk

Also prepended APM2 to existing values.  Added A2 to list to be
consistent with plane and copter

Also prepend APM2 to existing definitions

This is now part of the AP_Relay library

This resolves an issue in which the mission would not automatically
start from the beginning if it had previously been run to completion

Thanks to EmileC for spotting this

this may give smoother throttle response when the EKF is enabled

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>

This will enable in-flight magnetometer calibration to be inhibited unconditionally,
This is required for long balloon carriage flights where ground speed can be high
enough to put it into in-air state, but with very poor observability of magnetic field
states causing bad state estimates and heading offsets to develop over time.

The covariance matrix no longer has rows and columns artificially zeroed when in static
mode. Instead booleans indicating whether wind or magentic field state estimation is
active are used to:

a) Set the process noise on these states to zero to stop their variances from increasing
unchecked when not being updated, and
b) Turn off updates for these states when measurement fusion is being performed.

This reduces the likelihood of a badly conditioned covariance matrix forming
during static mode operation.

A filter divergence check has also been added that will declare the filter unhealthy if
position, velocity and magnetic field observations are all failing their innovation
consistency checks. This unhealthy status will persist for 10 seconds after the
condition clears.

AP_NavEKF: Remove unnecessary zeroing of wind covariances

Baro data can be used as it will still be valid during the launch and
is not a synthesised measurement.

Minor commit to kick off the auto build so AC3.2-rc1 will go out

this removes the debug code now that the issue has been found (it was
an old version of pymavlink causing the problem)

- added new mode for CH7/CH8 (#24, auto mission reset)
- changed mission.start() to mission.start_or_resume() in auto_init()