Give the sensor some time to adjust
Display the applied offset on the webpage, not just the parameter

Previously, it was hardcoded to 2000 ppm, which
remains the default value.

A soft reset of the SCD30 sensor, i.e.
calling `SCD30.reset()` was added in commit
fe023428 to force an instant update of
the measurement interval after bootup.

It seems, however, that the calibration
value is not stored if the sensor is
reset before carrying out at least one
measurement after calibration.
This breaks calibration with the current code,
because the ESP (and then the SCD30) is reset
directly after calibration.

* Trying 10s instead.
* Adding warning if timestep < 10s

Forced calibration should hopefully be more reliable now.

could be useful, but hasn't been integrated
yet.

Ongoing investigation showed that the sensor
does report 0 ppm only after startup, i.e.
in 'BOOTUP' state. We check for this condition
in `hasSensprSettled()` already, so there is no
need for this state anymore.

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.

Sometimes after a hard reset of the ESP the SCD30
needs a long time until returning the first measurement.
Resetting it after startup seems to fix this behaviour.

This state is showed with red waiting LEDs.
Previously, the state 'BOOTUP' with blue
waiting LEDs was used to show invalid measurements.

Use blue waiting LEDs now only during sensor
acclimatization.