AP_Math: quaternion: add contructor from float[4]

libraries/AP_Math/quaternion.h

@@ -40,6 +40,12 @@ public:
     {
     }
 
+    // setting constructor
+    Quaternion(const float _q[4]) :
+        q1(_q[0]), q2(_q[1]), q3(_q[2]), q4(_q[3])
+    {
+    }
+
     // function call operator
     void operator()(const float _q1, const float _q2, const float _q3, const float _q4)
     {