[math] pprz_isa: QNH functions
[air_data] use ABI for differential pressure, compute airspeed from it
[air_data] add qnh and amsl
[modules] make air_data a module
[airframes] add air_data module to some example airframes
[air_data] add TEMPERATURE ABI message and calc true airspeed

- removed airspeed_scale
- true airspeed (TAS) factor calculated from pressure and temp
  - if that is not available, tas_factor can be set manually (and could be used to replace airspeed_scale
[modules] USE_AIRSPEED_AIR_DATA flag

so you can let it calc the airspeed without pushing it to the state interface,
e.g. to use a different airspeed sensor for state interface and hence control
[messages][modules] add AIR_DATA message
[modules] initialize air_data variables
[modules] fix ABI subscription for BARO_DIFF
[modules] update air_data doc
[modules] air_data: init vars as unknown
[modules][messages] add AMSL message to air_data
Merge pull request #853 from paparazzi/air_data_improvements

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)
- send new AIR_DATA and AMSL messages