Add resistance property.

adafruit_max31865.py

@@ -194,19 +194,25 @@ class MAX31865:
         rtd >>= 1
         return rtd
 
+    @property
+    def resistance(self):
+        """Read the resistance of the RTD and return its value in Ohms."""
+        resistance = self.read_rtd()
+        resistance /= 32768
+        resistance *= self.ref_resistor
+        return resistance
+
     @property
     def temperature(self):
         """Read the temperature of the sensor and return its value in degrees
         Celsius.
         """
-        raw_reading = self.read_rtd()
-        raw_reading /= 32768
-        raw_reading *= self.ref_resistor
-
         # This math originates from:
         # http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
-        # To match the naming from the app note we tell lint to ignore the Z1-4 naming.
+        # To match the naming from the app note we tell lint to ignore the Z1-4
+        # naming.
         # pylint: disable=invalid-name
+        raw_reading = self.resistance
         Z1 = -_RTD_A
         Z2 = _RTD_A * _RTD_A - (4 * _RTD_B)
         Z3 = (4 * _RTD_B) / self.rtd_nominal