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
- subscribe to TEMPERATURE 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)
- calculate true airspeed (TAS) from equivalent airspeed (EAS) using a `tas_factor` (with default to 1.0)
- calculate `tas_factor` from ISA model and current pressure and temperature (enabled by default)

I think this is a better place to handle QNH than the qnh.xml module from #837 

QNH and TAS not tested in real flight so far...