This library proved to be just as useless for reliable IAQ as the
original BSEC library (swinging wildly based on nothing at all), so
revert back to tag 1.2 and ESP-IDF.
Implements an AQ calculation from [1], which should hopefully provide a
more stable, sensible value for the air quality.
This also adds a configurable gas ceiling level, needed for full
calibration and explained in the README.
[1] https://github.com/thstielow/raspi-bme680-iaq
The human text ratings are still a work-in-progress.
