[radio control] spektrum also usable for intermcu
[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
[gps] rename gps_has_been_good static var

we don't like shadowed declarations...
[modules] cleanup digital_cam/dc a bit

Some cleanup before starting to make it usable for rotorcrafts,
still only works for fixedwings (no functional changes for that)

TODO for #867:
- replace gps.utm with state interface
- reading waypoints in fixedwing/rotorcraft agnostic way
- and probably something else as well...
[NAV] Polygon Survey with orientation defined by a waypoint
[digital_cam] Uart camera control with thumbnails and simulation mode
[digital_cam_uart] Compile in master
[CATIA] Camera Application Triggering Image Acquisition
Merge pull request #847 from paparazzi/intermcu_rc

[radio control] spektrum also usable for intermcu

You have to add the radio_control subsystem to the ap target as well in order to get the RADIO_CONTROL_TYPE_H define to load the correct implementation header so we get the RADIO_CONTROL_NB_CHANNEL and channel names.
For spektrum you might have to define RADIO_CONTROL_NB_CHANNEL in your airframe file to something less than 10 to use it with the uart inter_mcu.
[codestyle] Apply PPRZ Code Style