Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/TutorialLinAlgSVDSolve.cpp

@@ -0,0 +1,14 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  MatrixXf A = MatrixXf::Random(3, 2);
+  cout << "Here is the matrix A:\n" << A << endl;
+  VectorXf b = VectorXf::Random(3);
+  cout << "Here is the right hand side b:\n" << b << endl;
+  cout << "The least-squares solution is:\n"
+       << A.bdcSvd(ComputeThinU | ComputeThinV).solve(b) << endl;
+}