Improve the workflow of calibrating the sensor values

[RonMcKay]

Currently it is quite cumbersome to calibrate the raw sensor values as one has to upload the sketch in CALIBRATION_MODE first, note the values in air and water, update the sketch and reupload it to the sensor. When trying to deploy a lot of these sensors this takes a lot of time. A nicer way would be to include a second button and an LED which could drive a small "calibration interface".