[radio_control] spektrum for nps/sim

Ocaml sim always uses the radio.xml file and sends ppm values in usec.
So for the time beeing you need to use the spektrum.xml radio file for the sim target if you really want to use the RC in the simple OCaml sim.

closes #503
[conf] update examples using spektrum for sim
[invariant] ignore warning 'cast-qual' locally
[units] add ft<->m conversion
update create_module
Merge pull request #851 from paparazzi/spektrum_sim

[radio_control] spektrum for nps/sim

OCaml sim always uses the radio.xml file and sends ppm values in usec.
So for the time beeing you need to use the spektrum.xml radio file for the sim target if you really want to use the RC in the simple OCaml sim.

closes #503
[generators] add type bool for gen_settings
[modules] add airspeed_ms45xx_i2c module
[modules] fixes for airspeed_ms45xx
[modules] fix ms45xx.airspeed calculation

Always take absolute value of differential pressure
[modules] airspeed_ms45xx: 1 psi default pressure range
[modules] ms45xx: add pressure scale and offset to settings

and make them saveable to airframe
[modules] ms45xx: persistent settings
[modules] ms45xx: set freq to 100Hz and add butterworth lowpass