Airspeed: implement pressure scaling factor.

- Scaling factor implemented for MEAS/Honeywell sensors
- Allows any sensors in the same family (with different scales) to be used
- Startup/default values are right for the standard sensors

airspeed: ETS airspeed now not affected by changes to scale

- ETS airspeed outputs a pre-scaled value so it's not affected
    by scaling
- Scale changes cause a ringbuffer flush to eliminate values
    that were calculated with incorrect scale/offset