Tutorial page 4: add some text, diversify examples.

Use \verbinclude for output text to disable syntax highlighting.
Give tables consistent look.

doc/examples/Tutorial_BlockOperations_colrow.cpp

@@ -1,15 +1,14 @@
 #include <Eigen/Dense>
 #include <iostream>
-using namespace Eigen;
 
 int main()
 {
-  MatrixXf m(3,3);
-  
+  Eigen::MatrixXf m(3,3);
   m << 1,2,3,
        4,5,6,
        7,8,9;
-     
-  std::cout << "2nd Row: "
-    << m.row(1) << std::endl;
+  std::cout << "2nd Row: " << m.row(1) << std::endl;
+  m.col(0) += m.col(2);
+  std::cout << "m after adding third column to first:\n";
+  std::cout << m << std::endl;
 }