AP_Airspeed: Allow setting of use-zero-offset in parameters

[peterbarker]

If a NMEA or SPD3X airspeed sensor is used as part of an AP_Periph node then the airspeed data returned to the autopilot is already calibrated. When used directly both of these sensors flag that a zero-offset should be used, but when the data is returned via uavcan's RawAirData packet we do not have this information.

Problems arise when these sensors return perfect-zeroes as readings for the entire calibration routine, as we then set offset to zero - and at that point the sensor is considered unhealthy and is not used.

This PR allows a user to set a bit indicating the data is already compensated - forcing SET_USE_ZERO

There are three other approaches I've considered:

• have the user set ARSPD_SKIP_CAL to 1 and ARSPD_OFFSET to 0.00001 and move on with life
• stop sending RawAirData in AP_Periph for sensors which use zero offset. Instead, send IndicatedAirSpeed or TrueAirspeed if we have a baro. This would also involve wrangling the airspeed values in ArduPilot somehow, and that is a difficult story to tell; AP_Airspeed would probably have to fake things up.
• have NMEA and SPD3X return a very, very small pressure all the time to avoid the calibration issue

[tridge]

easier to just accept zero value when skip cal is set