Fix set_mag_gain definition and add to the example

Adafruit_LSM303/LSM303.py

@@ -85,7 +85,7 @@ def read(self):
         mag = struct.unpack('>hhh', mag_raw)
         return (accel, mag)
 
-    def set_mag_gain(gain=LSM303_MAGGAIN_1_3):
+    def set_mag_gain(self,gain=LSM303_MAGGAIN_1_3):
         """Set the magnetometer gain.  Gain should be one of the following
         constants:
          - LSM303_MAGGAIN_1_3 = +/- 1.3 (default)

examples/simpletest.py

@@ -12,6 +12,8 @@
 # Create a LSM303 instance.
 lsm303 = Adafruit_LSM303.LSM303()
 
+lsm303.set_mag_gain(Adafruit_LSM303.LSM303_MAGGAIN_1_3)
+
 # Alternatively you can specify the I2C bus with a bus parameter:
 #lsm303 = Adafruit_LSM303.LSM303(busum=2)