get rid of using namespace Eigen in sample code

doc/examples/tut_arithmetic_add_sub.cpp

@@ -1,14 +1,12 @@
 #include <iostream>
 #include <Eigen/Dense>
 
-using namespace Eigen;
-
 int main()
 {
-  Matrix2d a;
+  Eigen::Matrix2d a;
   a << 1, 2,
        3, 4;
-  MatrixXd b(2,2);
+  Eigen::MatrixXd b(2,2);
   b << 2, 3,
        1, 4;
   std::cout << "a + b =\n" << a + b << std::endl;
@@ -16,7 +14,7 @@ int main()
   std::cout << "Doing a += b;" << std::endl;
   a += b;
   std::cout << "Now a =\n" << a << std::endl;
-  Vector3d v(1,2,3);
-  Vector3d w(1,0,0);
+  Eigen::Vector3d v(1,2,3);
+  Eigen::Vector3d w(1,0,0);
   std::cout << "-v + w - v =\n" << -v + w - v << std::endl;
 }