Calibration of CO2 sensor to custom value or offset option for CO2 sensors

[hapklaar]

The Sensirion SCD30 sensor allows for calibration of the sensor to a value other than 400ppm. This is helpful as our outside air is currently around 419ppm, so when you calibrate that to 400, the device always show a too low value.

Additionally, for sensors that don't support this, maybe an offset possibility could be added to allow for corrections if desired. Or even do an offset option for all CO2 sensors and leave the custom calibration for what it is.