heater: Move adc logic into Thermistor class

The Thermistor (and Linear) class should handle all the details of
reading the ADC values and converting them to temperatures.  So, move
that logic out of the Heater() class.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/pid_calibrate.py

@@ -62,7 +62,7 @@ class ControlAutoTune:
             self.pwm_samples.append((read_time + heater.PWM_DELAY, value))
             self.last_pwm = value
         self.heater.set_pwm(read_time, value)
-    def adc_callback(self, read_time, temp):
+    def temperature_callback(self, read_time, temp):
         self.temp_samples.append((read_time, temp))
         if self.heating and temp >= self.heater.target_temp:
             self.heating = False