Allow setting of EKF3 wind state from MAV_CMD_EXTERNAL_WIND_ESTIMATE

[peterbarker]

This is mostly @priseborough 's work, I've just tidied things up a bit.

• I've ensured the wind estimates replay correctly
• I've ensured the WIND mavlink message contains the wind estimate while EKF3 is in charge
• I've checked the XKF2[0].VW* messages to contain the supplied data
• the supplied wind estimates don't last long during a bog-standard Plane takeoff in SITL, the Plane replaces them quickly

[peterbarker]

• Try passing a nan in in the autotest suite for the accuracies and see what happens
• Check the sign is correct; set a wind estimate and a simulated speed to match, take-off and make sure estimates remain consistent

[peterbarker]

[peterbarker]

Before my most timestamp-fix patch:

After my most timestamp-fix patch:

[landswellsong]

Just in case, flew several times with this PR plucked over 4.5.7 (with DCM changes too) and it seems to work as intended. Didn't see any scary behaviour, can sieve through logs if there is anything in particular that can help testing.