Extract common interface for Airspeed Heading Messages

[mrksngl]

Airborne Velocity Messages are split into

• Airspeed Heading Messages, implemented by AirspeedHeadingMsg
• Velocity Over Ground Messages, implemented by VelocityOverGroundMsg

Yet, they have common information, in particular

• Intent Change Flag
• IFR Flag (only defined for V0 and V1)
• NUC_R (V0) resp. NAV_v (V1+)
• vertical rate
• diff from baro alt (called geoMinusBaro in the lib)
• turn indicator (for sake of completeness - only available in V0 and not implemented by the lib)

With a common AirborneVelocityMessage interface, code duplication in code using the lib can be greatly reduced.

[mrksngl]

See also #49 on NAC_v