Add microbit accelerometer subscription (#417)

* Add microbit accelerometer subscription

* fix docstring lint error

Author: WayneKeenan

bluezero/microbit.py

@@ -96,6 +96,7 @@ def __init__(self, device_addr, adapter_addr=None, **kwargs):
         self.user_btn_b_callback = None
         self.user_calibrate_cb = None
         self.uart_tx_cb = None
+        self.user_accel_cb = None
 
         # Micro:bit Characteristics
         # if accelerometer_service:
@@ -392,6 +393,22 @@ def accelerometer(self):
 
         return tools.bytes_to_xyz(accel_bytes)
 
+    def _decode_accel(self, *accel_values):
+        """Decode accelerometer values and pass on to user callback."""
+        self.user_accel_cb(*tools.bytes_to_xyz(accel_values[1]['Value']))
+
+    def subscribe_accelerometer(self, user_callback):
+        """
+        Execute user_callback on data being received on accelerometer service
+
+        :param user_callback: User callback method receiving the accelerometer
+            values
+        :return:
+        """
+        self.user_accel_cb = user_callback
+        self._accel_data.add_characteristic_cb(self._decode_accel)
+        self._accel_data.start_notify()
+
     @property
     def magnetometer(self):
         """