AP_GPS: added get_lag method to return expected lag from the GPS.

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -35,6 +35,8 @@ public:
     static const prog_char          _ublox_set_binary[];
     static const uint8_t            _ublox_set_binary_size;
 
+    float       get_lag() { return 0.5; }   // ublox lag is lower than the default 1second
+
 private:
     // u-blox UBX protocol essentials
 // XXX this is being ignored by the compiler #pragma pack(1)

libraries/AP_GPS/GPS.h

@@ -135,6 +135,8 @@ public:
         return _last_ground_speed_cm * 0.01;
     }
 
+    // the expected lag (in seconds) in the position and velocity readings from the gps
+    virtual float get_lag() { return 1.0; }
 
     // the time we got our last fix in system milliseconds
     uint32_t last_fix_time;