added support for pillars

some new helper methods/classes

sensors/radio/WiFiMeasurements.h

@@ -2,6 +2,7 @@
 #define WIFIMEASUREMENTS_H
 
 #include <vector>
+#include <algorithm>
 
 #include "WiFiMeasurement.h"
 
@@ -42,6 +43,14 @@ struct WiFiMeasurements {
 		}
 	}
 
+	/** get the oldest timestamp among all contained measurements */
+	Timestamp getOldestTS() const {
+		auto comp = [] (const WiFiMeasurement& m1, const WiFiMeasurement& m2) {return m1.getTimestamp() < m2.getTimestamp();};
+		auto it = std::max_element(entries.begin(), entries.end(), comp);
+		if (it == entries.end()) {throw Exception("no element found");}
+		return it->getTimestamp();
+	}
+
 
 	/** create a combination */
 	static WiFiMeasurements mix(const WiFiMeasurements& a, const WiFiMeasurements& b, float sec = 3) {