heater: Move Thermistor and Linear to their own files in extras/

Move the Thermistor code to a new thermistor.py module.  Move the
Linear code to a new adc_temperature.py module.  This simplifies the
heater.py code.

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

klippy/extras/adc_temperature.py

@@ -0,0 +1,45 @@
+# Obtain temperature using linear interpolation of ADC values
+#
+# Copyright (C) 2016-2018  Kevin O'Connor <kevin@koconnor.net>
+#
+# This file may be distributed under the terms of the GNU GPLv3 license.
+
+SAMPLE_TIME = 0.001
+SAMPLE_COUNT = 8
+REPORT_TIME = 0.300
+
+# Linear style conversion chips calibrated with two temp measurements
+class Linear:
+    def __init__(self, config, params):
+        adc_voltage = config.getfloat('adc_voltage', 5., above=0.)
+        ppins = config.get_printer().lookup_object('pins')
+        self.mcu_adc = ppins.setup_pin('adc', config.get('sensor_pin'))
+        self.mcu_adc.setup_adc_callback(REPORT_TIME, self.adc_callback)
+        self.temperature_callback = None
+        slope = (params['t2'] - params['t1']) / (params['v2'] - params['v1'])
+        self.gain = adc_voltage * slope
+        self.offset = params['t1'] - params['v1'] * slope
+    def setup_minmax(self, min_temp, max_temp):
+        adc_range = [self.calc_adc(min_temp), self.calc_adc(max_temp)]
+        self.mcu_adc.setup_minmax(SAMPLE_TIME, SAMPLE_COUNT,
+                                  minval=min(adc_range), maxval=max(adc_range))
+    def setup_callback(self, temperature_callback):
+        self.temperature_callback = temperature_callback
+    def get_report_time_delta(self):
+        return REPORT_TIME
+    def adc_callback(self, read_time, read_value):
+        temp = read_value * self.gain + self.offset
+        self.temperature_callback(read_time + SAMPLE_COUNT * SAMPLE_TIME, temp)
+    def calc_adc(self, temp):
+        return (temp - self.offset) / self.gain
+
+Sensors = {
+    "AD595": { 't1': 25., 'v1': .25, 't2': 300., 'v2': 3.022 },
+}
+
+def load_config(config):
+    # Register default sensors
+    pheater = config.get_printer().lookup_object("heater")
+    for sensor_type, params in Sensors.items():
+        func = (lambda config, params=params: Linear(config, params))
+        pheater.add_sensor(sensor_type, func)