some improvements on tutorials

samples/cpp/tutorial_code/ml/introduction_to_pca/introduction_to_pca.cpp

@@ -94,12 +94,16 @@ double getOrientation(const vector<Point> &pts, Mat &img)
 /**
  * @function main
  */
-int main(int, char** argv)
+int main(int argc, char** argv)
 {
 //! [pre-process]
     // Load image
-//    Mat src = imread("pca_test1.jpg");
-    Mat src = imread(argv[1]);
+    String imageName("../data/pca_test1.jpg"); // by default
+    if (argc > 1)
+    {
+        imageName = argv[1];
+    }
+    Mat src = imread( imageName );
 
     // Check if image is loaded successfully
     if(!src.data || src.empty())