fix some samples colliding with std::beta

also add comments explaining the change

samples/cpp/tutorial_code/ImgProc/BasicLinearTransforms.cpp

@@ -8,7 +8,10 @@
 #include "opencv2/highgui.hpp"
 #include <iostream>
 
-using namespace std;
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
+using std::cin;
+using std::cout;
+using std::endl;
 using namespace cv;
 
 /**

samples/cpp/tutorial_code/ImgProc/changing_contrast_brightness_image/changing_contrast_brightness_image.cpp

@@ -2,7 +2,7 @@
 #include "opencv2/imgcodecs.hpp"
 #include "opencv2/highgui.hpp"
 
-using namespace std;
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
 using namespace cv;
 
 namespace