added getAbsoluteHumidity from Adafruit sgp30test

stempayload/Payload_OK_SGP30/Payload_OK_SGP30.ino

@@ -28,6 +28,7 @@ void blink_setup();
 void blink(int length);
 int read_analog();
 void led_set(int ledPin, bool state);
+uint32_t getAbsoluteHumidity(float temperature, float humidity);
 
 int first_time = true;
 int first_read = true;
@@ -423,3 +424,10 @@ int read_analog()
 #endif
     return(sensorValue); 
 }
+
+uint32_t getAbsoluteHumidity(float temperature, float humidity) {
+    // approximation formula from Sensirion SGP30 Driver Integration chapter 3.15
+    const float absoluteHumidity = 216.7f * ((humidity / 100.0f) * 6.112f * exp((17.62f * temperature) / (243.12f + temperature)) / (273.15f + temperature)); // [g/m^3]
+    const uint32_t absoluteHumidityScaled = static_cast<uint32_t>(1000.0f * absoluteHumidity); // [mg/m^3]
+    return absoluteHumidityScaled;
+}