graphical exception [temporary solution]

performance fixes
minor changes

sensors/radio/WiFiProbabilityGrid.h

@@ -56,6 +56,12 @@ public:
 		double prob = 1;
 		int numMatchingAPs = 0;
 
+		// after some runtime, check whether the wifi timestamps make sense
+		// those must not be zero, otherwise something is wrong!
+		if (!obs.entries.empty()) {
+			Assert::isFalse(curTime.ms() > 10000 && obs.entries.front().ts.isZero(), "WiFiMeasurement timestamp is 0. this does not make sense...");
+		}
+
 		// process each observed measurement
 		for (const WiFiMeasurement& measurement : obs.entries) {