fixed some compiler warnings

added equality checks to sensor-data classes
more robust sensor reader [fixed some issues]
added support for gps
added support for compass
added sensor-data-writer
added test-cases
minor changes

sensors/radio/WiFiMeasurement.h

@@ -20,7 +20,7 @@ private:
 	float rssi;
 
     /** OPTIONAL. frequence the signal was received */
-    float freq;
+	float freq = NAN;
 
 	/** OPTIONAL. timestamp the measurement was recorded at */
 	Timestamp ts;
@@ -28,7 +28,7 @@ private:
 public:
 
 	/** ctor */
-	WiFiMeasurement(const AccessPoint& ap, const float rssi) : ap(ap), rssi(rssi) {
+	WiFiMeasurement(const AccessPoint& ap, const float rssi) : ap(ap), rssi(rssi), freq(NAN) {
 		;
 	}