AP_AHRS: publish have_velocity_source in the backend estimates

Add have_velocity_source to AP_AHRS_Backend::Estimates: true if the
backend has a ground-velocity source that is independent of airspeed
(e.g. GPS), so the value may be used for synthetic airspeed without
circularity.  DCM publishes have_gps().  It may be true while
velocity_NED_valid is false (e.g. DCM with a 2D GPS fix).

libraries/AP_AHRS/AP_AHRS_Backend.h

@@ -111,6 +111,12 @@ public:
         // a ground velocity in meters/second, North/East/Down
         Vector3f velocity_NED;
         bool velocity_NED_valid;
+        // true if this backend has a ground-velocity source which is
+        // independent of airspeed (e.g. GPS or an external INS); such
+        // a velocity may be used for wind estimation or synthetic
+        // airspeed without circularity.  This may be true while
+        // velocity_NED_valid is false (e.g. DCM with a 2D GPS fix):
+        bool have_velocity_source;
         // return a ground velocity in meters/second, North/East/Down
         bool get_velocity_NED(Vector3f &vel) const WARN_IF_UNUSED {
             if (!velocity_NED_valid) {

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -204,6 +204,8 @@ void AP_AHRS_DCM::get_results(AP_AHRS_Backend::Estimates &results)
     results.accel_ef = _accel_ef;
 
     results.velocity_NED_valid = get_velocity_NED(results.velocity_NED);
+    // DCM's airspeed-independent velocity source is the GPS:
+    results.have_velocity_source = have_gps();
 
     // ground velocity estimate in meters/second, in North/East order
     // note: velocity_NE is significantly different to results.velocity_NED.xy()!