AP_Gripper: move gripper up to AP_Vehicle

also make the singleton return a reference rather than a pointer

libraries/AP_Gripper/AP_Gripper.cpp

@@ -168,9 +168,9 @@ PASS_TO_BACKEND(grabbed)
 
 namespace AP {
 
-AP_Gripper *gripper()
+AP_Gripper &gripper()
 {
-    return AP_Gripper::get_singleton();
+    return *AP_Gripper::get_singleton();
 }
 
 };

libraries/AP_Gripper/AP_Gripper.h

@@ -88,7 +88,7 @@ private:
 };
 
 namespace AP {
-    AP_Gripper *gripper();
+    AP_Gripper &gripper();
 };
 
 #endif  // AP_GRIPPER_ENABLED