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.