Better API for airborne/ground status

[fixje]

Currently, airborne/ground status in some messages can be checked with the following methods:

public boolean isOnGround()
public boolean isAirborne()

As the status might be unknown, both can return false. A solution which avoids cross references in the javadoc and implicit knowledge would just use a single method which can return null for an unknown status:

/**
 * @return true if airborne, false if on ground, null if unknown
 */
public Boolean isAirborne();

This needs to be changed in

• AllCallReply
• AltitudeReply
• CommBAltitudeReply
• IdentifyReply
• CommBIdentifyReply

This is an incompatible API change for the next major release