this allows us to use the MS5611 barometer at its full 100Hz sample
rate (80Hz for pressure, 20Hz for temperature). The pressure and
temperature values are averaged between reads without adding any
latency. Previously the driver would throw away values between
readings
This also fixes a race condition in reading from the SPI bus that
could lead to bad values from the barometer
this allows the barometer driver to calibrate and return altitude and
climb rate values. This will be used by the AHRS drift correction code
for vertical velocity
The climb rate uses a 5 point average filter
Andrew Tridgell
Pat Hickey
Pat Hickey