* 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.