refactored OpticalFlowDual_TVL1:

* added DenseOpticalFlow interface
* moved OpticalFlowDual_TVL1 to src folder

samples/cpp/tvl1_optical_flow.cpp

@@ -173,10 +173,10 @@ int main(int argc, const char* argv[])
     }
 
     Mat_<Point2f> flow;
-    OpticalFlowDual_TVL1 tvl1;
+    Ptr<DenseOpticalFlow> tvl1 = createOptFlow_DualTVL1();
 
     const double start = (double)getTickCount();
-    tvl1(frame0, frame1, flow);
+    tvl1->calc(frame0, frame1, flow);
     const double timeSec = (getTickCount() - start) / getTickFrequency();
     cout << "calcOpticalFlowDual_TVL1 : " << timeSec << " sec" << endl;