Added double_tap to express class (#23)

Has a warning if the built in LIS3DH library is too old.

Author: kattni

adafruit_circuitplayground/express.py

@@ -118,6 +118,36 @@ def __init__(self):
         self._lis3dh = adafruit_lis3dh.LIS3DH_I2C(self._i2c, address=0x19)
         self._lis3dh.range = adafruit_lis3dh.RANGE_8_G
 
+        # Initialise tap:
+        self._lis3dh.set_tap(2, 18, time_limit=4, time_latency=17, time_window=110)
+        self._last_tap = False
+
+    @property
+    def double_tap(self):
+        """True once after a double tap.
+
+        .. image :: /_static/accelerometer.jpg
+          :alt: Accelerometer
+
+        Quickly tap the CPX twice to double-tap.
+
+        .. code-block:: python
+
+          from adafruit_circuitplayground.express import cpx
+
+          while True:
+              if cpx.double_tap:
+                  print("Double tap!")
+        """
+        try:
+            tapped = self._lis3dh.tapped
+            first_double_tap = tapped and not self._last_tap
+            self._last_tap = tapped
+            return first_double_tap
+        except AttributeError:
+            raise RuntimeError("Oops! You need a newer version of CircuitPython "
+                               "(2.2.0 or greater) to use this feature.")
+
     @property
     def acceleration(self):
         """Obtain data from the x, y and z axes.
@@ -172,7 +202,7 @@ def shake(self, shake_threshold=30):
             return self._lis3dh.shake(shake_threshold=shake_threshold)
         except AttributeError:
             raise RuntimeError("Oops! You need a newer version of CircuitPython "
-                               "(2.2.0 or greater) to use cpx.shake.")
+                               "(2.2.0 or greater) to use this feature.")
 
     @property
     def touch_A1(self): # pylint: disable=invalid-name