Support for specifying ambient temperature thermistor

[Luro02]

Finding the ambient temperature after the printer has been used can be difficult, because at some point close to the ambient temperature, the printer will cool down very slowly.

Some have a heated chamber or an ambient temperature installed.

What should be possible?

• [ ] Specify a sensor in the config, they are defined in this section https://www.klipper3d.org/Config_Reference.html#temperature_sensor

What might be possible?

• [ ] If the bed is cold, one could use it to read the ambient temperature
• [ ] Use the current raspberry pi temperature as ambient? (not sure if that is reliable)

How much does the real ambient temperature matter?

At the time of writing this, the code is using the real ambient temperature for two things:

1. to calculate the asymptotic temperature (if I remember correctly, there is not much of a difference between fixing it to 25°C and using the actual temperature like 17°C)
2. to calculate the ambient_xfer_coeff

The calculation of the ambient_xfer_coeff is flawed and might be switched to a different formula.

[chrisheib]

Pi temperature is ~15C above ambient in idle for me, so probably not the greatest source of ambient temp.