Plane: log currently used airspeed

this logs whatever airspeed is being used for flight

ArduPlane/Log.cpp

@@ -109,11 +109,15 @@ struct PACKED log_Control_Tuning {
     int16_t throttle_out;
     int16_t rudder_out;
     int16_t throttle_dem;
+    float airspeed_estimate;
 };
 
 // Write a control tuning packet. Total length : 22 bytes
 void Plane::Log_Write_Control_Tuning()
 {
+    float est_airspeed = 0;
+    ahrs.airspeed_estimate(&est_airspeed);
+    
     struct log_Control_Tuning pkt = {
         LOG_PACKET_HEADER_INIT(LOG_CTUN_MSG),
         time_us         : AP_HAL::micros64(),
@@ -123,7 +127,8 @@ void Plane::Log_Write_Control_Tuning()
         pitch           : (int16_t)ahrs.pitch_sensor,
         throttle_out    : (int16_t)SRV_Channels::get_output_scaled(SRV_Channel::k_throttle),
         rudder_out      : (int16_t)SRV_Channels::get_output_scaled(SRV_Channel::k_rudder),
-        throttle_dem    : (int16_t)SpdHgt_Controller->get_throttle_demand()
+        throttle_dem    : (int16_t)SpdHgt_Controller->get_throttle_demand(),
+        airspeed_estimate : est_airspeed
     };
     DataFlash.WriteBlock(&pkt, sizeof(pkt));
 }
@@ -367,7 +372,7 @@ const struct LogStructure Plane::log_structure[] = {
     { LOG_STARTUP_MSG, sizeof(log_Startup),         
       "STRT", "QBH",         "TimeUS,SType,CTot" },
     { LOG_CTUN_MSG, sizeof(log_Control_Tuning),     
-      "CTUN", "Qcccchhh",    "TimeUS,NavRoll,Roll,NavPitch,Pitch,ThrOut,RdrOut,ThrDem" },
+      "CTUN", "Qcccchhhf",    "TimeUS,NavRoll,Roll,NavPitch,Pitch,ThrOut,RdrOut,ThrDem,Aspd" },
     { LOG_NTUN_MSG, sizeof(log_Nav_Tuning),         
       "NTUN", "Qfcccfff",  "TimeUS,WpDist,TargBrg,NavBrg,AltErr,XT,XTi,ArspdErr" },
     { LOG_SONAR_MSG, sizeof(log_Sonar),