Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/TutorialLinAlgExSolveLDLT.cpp

@@ -0,0 +1,15 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  Matrix2f A, b;
+  A << 2, -1, -1, 3;
+  b << 1, 2, 3, 1;
+  cout << "Here is the matrix A:\n" << A << endl;
+  cout << "Here is the right hand side b:\n" << b << endl;
+  Matrix2f x = A.ldlt().solve(b);
+  cout << "The solution is:\n" << x << endl;
+}