fixed some issues

some new helper methods
added listener-support to offline-reader

sensors/radio/WiFiMeasurements.h

@@ -22,6 +22,16 @@ struct WiFiMeasurements {
 		return res;
 	}
 
+	/** get the measurements for the given MAC [if available] otherwise null */
+	WiFiMeasurement* getForMac(const MACAddress& mac) {
+		for (WiFiMeasurement& m : entries) {
+			if (m.getAP().getMAC() == mac) {
+				return &m;
+			}
+		}
+		return nullptr;
+	}
+
 };
 
 #endif // WIFIMEASUREMENTS_H