this changes the barometer calculations to floating point. On a MS5611
this is actually about twice as fast as the previous 64 bit
calculations, but gains us more accuracy as we are able to take
advantage of sub-bit precision when we average over 8 samples.
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
Randy Mackay
Pat Hickey