Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/DenseBase_middleCols_int.cpp

@@ -0,0 +1,14 @@
+#include <Eigen/Core>
+#include <iostream>
+
+using namespace Eigen;
+using namespace std;
+
+int main(void) {
+  int const N = 5;
+  MatrixXi A(N, N);
+  A.setRandom();
+  cout << "A =\n" << A << '\n' << endl;
+  cout << "A(1..3,:) =\n" << A.middleCols(1, 3) << endl;
+  return 0;
+}