co2_sensor: Do not report new data in all cases

Log every measurement to the serial console, but
return only `true` in `processData()`(thus starting
further processing like CSV, MQTT, LORAWAN) if
the data is reliable (stable measurements, CO2 > 0)
or the sensor measures too low CO2 values (< 250).

The latter condition should be reported, because
the user can then initiate a manual calibration procedure.

ampel-firmware/co2_sensor.cpp

@@ -289,8 +289,10 @@ namespace sensor {
     }
 
     showState();
-
-    return freshData;
+    
+    // Report data for further processing only if the data is reliable 
+    // (state 'READY') or manual calibration is necessary (state 'NEEDS_CALIBRATION').
+    return freshData && (current_state == READY || current_state == NEEDS_CALIBRATION);
   }
 
   /*****************************************************************