[FR] I2C IR Thermometer (MLX90614) support

[altimmons]

Feature Request
It is difficult to get an accurate bed temp while avoiding putting the thermistor on top. And if you put it underneath it over reports. And as some users have shown, bed temp can vary place to place.

I am suggesting adding an IR Thermometer to my carriage that will measure the bed temperature that communicates with I2C. I think this may be the best way to come up with an average bed temperature, and is a better, non-interfering way to look at surface temperature

MLX90614 makes a series of cheap IR Thermometers, some are able to be "lensed" to reduce interference from the hot-ends if on the carriage.

My feature request is support for this method of measuring temperature over I2C and integration into PID temperature control of the bed.

I am not sure how to pull this value from I2C and plug it into the PID temperature control. There are libraries for I2C communication with the MLX and I could figure some of this out. But I suggest this will be a standard part of the carriage electronics package down the road as this technology matures.

It should be pretty easy. You would still need a bed thermistor to prevent overheating.