Use fixed ambient_temperature on first gas reading, it's nearly irrelevant

Note: on subsequent gas measurements the last temperature instead of 25 degree C will be used.
Nearly irrelevant because this happens to return value of _calc_heater_resistance(200):

ambient_temperature = 30 -> int(84.9119287004)
ambient_temperature = 25 -> int(84.9119068473)
ambient_temperature = 20 -> int(84.9118849941)
ambient_temperature = 10 -> int(84.9118412877)

library/bme680/__init__.py

@@ -51,6 +51,7 @@ class BME680(BME680Data):
         self._get_calibration_data()
 
         self.set_temp_offset(0)
+        self.ambient_temperature = 25 # only for gas heater temperature, nearly irrelevant
 
     def _get_calibration_data(self):
         """Retrieve the sensor calibration data and store it in .calibration_data."""