Air data improvements and conversion to module

Converts air_data to a module and adds:
- math functions for QNH, etc. from #837 
- subscribe to BARO_DIFF via ABI
- calculate airspeed from differential pressure if enabled (default)
- calculate QNH from abs pressure and altitude (enabled by default)
- calculate AMSL from baro pressure and QNH (disabled by default)

I think this is a better place to handle QNH than the qnh.xml module from #837 

Calculation of airspeed from differential pressure could probably be improved by not assuming a "fixed" quadratic scale and instead using the pressure and altitude information to get the air density.