this allows the use of an external temperature sensor for calibration
purposes, such as the sensor built in to the digital airspeed sensor.

The main affect this has is on the EAS2TAS calculation

The get_calibration_temperature() is used to choose either an external
temperature or an internal one. If an internal one is used then it is
clamped at no higher than 25 degrees C, to prevent hot electronics
on startup affecting altitude scaling and EAS2TAS