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
the averaging array was using 16 bit numbers, but we are storing
numbers with 19 significant bits. That caused overflow at high
altitude, and some very interesting altitude graphs!
Thanks to Michael Oborne for spotting this in a log
Andrew Tridgell
Pat Hickey