zero() reset the kalman filter

lib/Controller.js

@@ -99,6 +99,16 @@ Controller.prototype.hover = function() {
     this.go({x: this._state.x, y: this._state.y, z: this._state.z, yaw: this._state.yaw});
 }
 
+/*
+ * Reset the kalman filter to its base state (default is x:0, y:0, yaw:0).
+ *
+ * This is especially usefull to set mark the drone position as the starting position
+ * after takeoff.
+ */
+Controller.prototype.zero = function() {
+    this._ekf.reset();
+}
+
 /*
  * Move forward (direction faced by the front camera) by the given
  * distance (in meters).

tests/unit/test-Controller.js

@@ -107,5 +107,13 @@ test('Controller', {
 
         ctrl.left(1, cb);
         assert(ctrl.right.calledWith(-1, cb));
+    },
+
+    'zero reset the kalman filter': function() {
+        var ctrl    = new autonomy.Controller(this.mockClient);
+        sinon.spy(ctrl._ekf, 'reset');
+
+        ctrl.zero();
+        assert(ctrl._ekf.reset.calledOnce);
     }
 });