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.

LED Effects : small refactor, to allow breathing to be disabled (remove MIN_BRIGHTNESS or set it to MAX_BRIGHTNESS).