SITL: make C_TO_KELVIN a function macro; create KELVIN_TO_C

These are in celsius

libraries/SITL/SIM_Airspeed_DLVR.cpp

@@ -69,5 +69,5 @@ void SITL::Airspeed_DLVR::update(const class Aircraft &aircraft)
     AP_Baro::SimpleAtmosphere(sim_alt * 0.001f, sigma, delta, theta);
 
     // To Do: Add a sensor board temperature offset parameter
-    temperature = (SSL_AIR_TEMPERATURE * theta - C_TO_KELVIN) + 25.0;
+    temperature = (KELVIN_TO_C(SSL_AIR_TEMPERATURE * theta)) + 25.0;
 }

libraries/SITL/SIM_BattMonitor_SMBus.cpp

@@ -17,7 +17,7 @@ SITL::SIM_BattMonitor_SMBus::SIM_BattMonitor_SMBus() :
     add_block("Device Name", SMBusBattDevReg::DEVICE_NAME, SITL::I2CRegisters::RegMode::RDONLY);
     add_register("Manufacture Data", SMBusBattDevReg::MANUFACTURE_DATA, SITL::I2CRegisters::RegMode::RDONLY);
 
-    set_register(SMBusBattDevReg::TEMP, (int16_t)((15 + C_TO_KELVIN)*10));
+    set_register(SMBusBattDevReg::TEMP, (int16_t)((C_TO_KELVIN(15))*10));
      // see update for voltage
      // see update for current
      // TODO: remaining capacity

libraries/SITL/SIM_Frame.cpp

@@ -319,7 +319,7 @@ float Frame::get_air_density(float alt_amsl) const
     AP_Baro::SimpleAtmosphere(alt_amsl * 0.001f, sigma, delta, theta);
 
     const float air_pressure = SSL_AIR_PRESSURE * delta;
-    return air_pressure / (ISA_GAS_CONSTANT * (C_TO_KELVIN + model.refTempC));
+    return air_pressure / (ISA_GAS_CONSTANT * (C_TO_KELVIN(model.refTempC)));
 }
 
 /*

libraries/SITL/SIM_MS5525.cpp

@@ -72,6 +72,6 @@ void MS5525::get_pressure_temperature_readings(float &P_Pa, float &Temp_C)
     AP_Baro::SimpleAtmosphere(sim_alt * 0.001f, sigma, delta, theta);
 
     // To Do: Add a sensor board temperature offset parameter
-    Temp_C = (SSL_AIR_TEMPERATURE * theta - C_TO_KELVIN) + 25.0;
+    Temp_C = (KELVIN_TO_C(SSL_AIR_TEMPERATURE * theta)) + 25.0;
     P_Pa = AP::sitl()->state.airspeed_raw_pressure[0];
 }

libraries/SITL/SIM_MS5611.cpp

@@ -114,7 +114,7 @@ void MS5611::get_pressure_temperature_readings(float &P_Pa, float &Temp_C)
     AP_Baro::SimpleAtmosphere(sim_alt * 0.001f, sigma, delta, theta);
     P_Pa = SSL_AIR_PRESSURE * delta;
 
-    Temp_C = (SSL_AIR_TEMPERATURE * theta - C_TO_KELVIN) + AP::sitl()->temp_board_offset;
+    Temp_C = KELVIN_TO_C(SSL_AIR_TEMPERATURE * theta) + AP::sitl()->temp_board_offset;
 
     // TO DO add in temperature adjustment by inheritting from AP_Baro_SITL_Generic?
     // AP_Baro_SITL::temperature_adjustment(P_Pa, Temp_C);

libraries/SITL/SIM_SerialRangeFinder.cpp

@@ -48,7 +48,7 @@ void SerialRangeFinder::send_temperature()
     // Use the simple underwater model to get temperature
     float rho, delta, theta;
     AP_Baro::SimpleUnderWaterAtmosphere(-0.5 * 0.001, rho, delta, theta); // get simulated temperature for 0.5m depth
-    const float temperature = Aircraft::rand_normal(SSL_AIR_TEMPERATURE * theta - C_TO_KELVIN, 1); // FIXME pick a stddev based on data sheet
+    const float temperature = Aircraft::rand_normal(KELVIN_TO_C(SSL_AIR_TEMPERATURE * theta), 1); // FIXME pick a stddev based on data sheet
 
     uint8_t data[255];
     const uint32_t packetlen = packet_for_temperature(temperature,