AP_Baro: enable use of pressure correction

this allows for temperature correction of baro pressure

libraries/AP_Baro/AP_Baro.cpp

@@ -235,7 +235,8 @@ void AP_Baro::update_calibration()
 {
     for (uint8_t i=0; i<_num_sensors; i++) {
         if (healthy(i)) {
-            sensors[i].ground_pressure.set(get_pressure(i));
+            float corrected_pressure = get_pressure(i) + sensors[i].p_correction;
+            sensors[i].ground_pressure.set(corrected_pressure);
         }
 
         // don't notify the GCS too rapidly or we flood the link
@@ -568,13 +569,13 @@ void AP_Baro::update(void)
                 sensors[i].ground_pressure = sensors[i].pressure;
             }
             float altitude = sensors[i].altitude;
+            float corrected_pressure = sensors[i].pressure + sensors[i].p_correction;
             if (sensors[i].type == BARO_TYPE_AIR) {
-                float pressure = sensors[i].pressure + sensors[i].p_correction;
-                altitude = get_altitude_difference(sensors[i].ground_pressure, pressure);
+                altitude = get_altitude_difference(sensors[i].ground_pressure, corrected_pressure);
             } else if (sensors[i].type == BARO_TYPE_WATER) {
                 //101325Pa is sea level air pressure, 9800 Pascal/ m depth in water.
                 //No temperature or depth compensation for density of water.
-                altitude = (sensors[i].ground_pressure - sensors[i].pressure) / 9800.0f / _specific_gravity;
+                altitude = (sensors[i].ground_pressure - corrected_pressure) / 9800.0f / _specific_gravity;
             }
             // sanity check altitude
             sensors[i].alt_ok = !(isnan(altitude) || isinf(altitude));

libraries/AP_Baro/AP_Baro.h

@@ -63,6 +63,10 @@ public:
     float get_temperature(void) const { return get_temperature(_primary); }
     float get_temperature(uint8_t instance) const { return sensors[instance].temperature; }
 
+    // get pressure correction in Pascal. Divide by 100 for millibars or hectopascals
+    float get_pressure_correction(void) const { return get_pressure_correction(_primary); }
+    float get_pressure_correction(uint8_t instance) const { return sensors[instance].p_correction; }
+    
     // accumulate a reading on sensors. Some backends without their
     // own thread or a timer may need this.
     void accumulate(void);