offset variables have mixed and unconventional signs #140

I have recently tried to calibrate a number of sensors, and used the AIR-1 offset entity concept in some home-assembled ESP8288 sensors (Si7021, BME280). I have also been looking at the calibration instructions for Davis weather stations.

I found:

• Davis: corrected = raw + offset
• AIR-1 Temperature and Humidity: corrected = raw - offset
• AIR-1 Pressure: corrected = raw + offset

Offsets to variables should have a consistent sign/formula. Calling something an offset is fine, and I expected it to be added. I treat Davis's approach as having more weight and longstanding history in the broader community.

Therefore, I think the AIR-1 code should be changed to to be corrected = raw + offset.

Probably there is some cool esphome trick to flip the sign of the variable on first boot with new code, with an aux variable that's persisted, and that can be gc'd after a year.