fixed some issues with stats::variance

fixed umbrella header for stats
added error-feedback to wifi optimizers
improved logging for wifi optimizers
adjusted calling-API for wifi-optimizers

sensors/radio/setup/WiFiFingerprints.h

@@ -13,6 +13,8 @@
  */
 class WiFiFingerprints {
 
+	static constexpr const char* name = "Fingerprints";
+
 private:
 
 //	/** the file to save the calibration model to */
@@ -96,6 +98,8 @@ public:
 
 		}
 
+		Log::add(name, "loaded " + std::to_string(fingerprints.size()) + " fingerprints");
+
 		inp.close();
 
 	}