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The SCD30 sensor has a response time of 20 s to reach
67 % of its final reading. The first measurement
can thus be unreliable, if the environment changed
shortly prior to startup. Set measurement interval to the
shortest possible value (2 s) and wait then, until the
measurements have stabilized. Show a blue led effect in
the meanwhile.

* split state PREPARECALIBRATION into
PREPARECALIBRATION_INSTABLE (co2 measurements are changing
too much to start calibration) and

PREPARECALIBRATION_STABLE (co2 measurements are stable,
now waiting for 60 stable measurements)

* Handle all possible LED signal states in showState()
* drop now unused `waiting_color`

This removes the superfluous bool `should_calibrate`.

Previously, the control flow was determined by a mixture of
`if-else`-constructes and premature `return` statements,
which made it hard to understand which case occurs when.

* define sensor states
* log every change of state to serial

There is no new functionality added yet.