Replace ADCChannel.ReadTemperature() with a simple ReadTemperature
function.
Not all chips will have a temperature sensor that is read by sampling an
ADC channel. The replacement ReadTemperature is simpler and more generic
to other chip families.
This breaks chips that were relying on the previous ReadTemperature
method. I hope it won't break a lot of existing code. If it does, a
fallback can be added.
