Baro: make get_altitude_difference const

libraries/AP_Baro/AP_Baro.cpp

@@ -132,7 +132,7 @@ void AP_Baro::update_calibration()
 
 // return altitude difference in meters between current pressure and a
 // given base_pressure in Pascal
-float AP_Baro::get_altitude_difference(float base_pressure, float pressure)
+float AP_Baro::get_altitude_difference(float base_pressure, float pressure) const
 {
     float ret;
 #if HAL_CPU_CLASS <= HAL_CPU_CLASS_16

libraries/AP_Baro/AP_Baro.h

@@ -43,7 +43,7 @@ public:
 
     // get altitude difference in meters relative given a base
     // pressure in Pascal
-    float        get_altitude_difference(float base_pressure, float pressure);
+    float get_altitude_difference(float base_pressure, float pressure) const;
 
     // get scale factor required to convert equivalent to true airspeed
     float        get_EAS2TAS(void);