corrected some wrong units (used in airspeed calculation)

apps/systemlib/conversions.c

@@ -150,5 +150,5 @@ void quat2rot(const float Q[4], float R[9])
 
 float get_air_density(float static_pressure, float temperature_celsius)
 {
-	return static_pressure / (CONSTANTS_AIR_GAS_CONST * (temperature_celsius + CONSTANTS_ABSOLUTE_NULL_KELVIN));
+	return static_pressure / (CONSTANTS_AIR_GAS_CONST * (temperature_celsius - CONSTANTS_ABSOLUTE_NULL_CELSIUS));
 }

apps/systemlib/conversions.h

@@ -44,10 +44,10 @@
 #include <float.h>
 #include <stdint.h>
 
-#define CONSTANTS_ONE_G				9.80665f
-#define CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C	1.225f
-#define CONSTANTS_AIR_GAS_CONST 8.31432f
-#define CONSTANTS_ABSOLUTE_NULL_KELVIN 273.15f
+#define CONSTANTS_ONE_G						9.80665f		// m/s^2
+#define CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C	1.225f			// kg/m^3
+#define CONSTANTS_AIR_GAS_CONST				287.1f 			// J/(kg * K)
+#define CONSTANTS_ABSOLUTE_NULL_CELSIUS		-273.15f		// °C
 
 __BEGIN_DECLS