AP_Airspeed: Add support for new message

[IamPete1]

This adds support for the new airspeed message. It does not remove the old named value float, maybe we should have them both until we get GCS support.

We need a similar message for baro and then we can remove the three scaled pressure messages.

[IamPete1]

rebased.

[tridge]

AIRSPEED_SENSOR_FLAGS is not defined in the XML

[IamPete1]

Rebased and moved sending from airspeed lib to GCS MAVLINK. Tested manually that we get the message and it cycles through sensors.