AP_Baro: add get_air_density_ratio

libraries/AP_Baro/AP_Baro.cpp

@@ -208,6 +208,12 @@ float AP_Baro::get_EAS2TAS(void)
     return _EAS2TAS;
 }
 
+// return air density / sea level density - decreases in altitude
+float AP_Baro::get_air_density_ratio(void)
+{
+    return 1.0f/(sq(get_EAS2TAS()));
+}
+
 // return current climb_rate estimeate relative to time that calibrate()
 // was called. Returns climb rate in meters/s, positive means up
 // note that this relies on read() being called regularly to get new data

libraries/AP_Baro/AP_Baro.h

@@ -79,6 +79,9 @@ public:
     // get scale factor required to convert equivalent to true airspeed
     float get_EAS2TAS(void);
 
+    // get air density / sea level density - decreases in altitude
+    float get_air_density_ratio(void);
+
     // get current climb rate in meters/s. A positive number means
     // going up
     float get_climb_rate(void);