Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/TutorialLinAlgSelfAdjointEigenSolver.cpp

@@ -0,0 +1,17 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  Matrix2f A;
+  A << 1, 2, 2, 3;
+  cout << "Here is the matrix A:\n" << A << endl;
+  SelfAdjointEigenSolver<Matrix2f> eigensolver(A);
+  if (eigensolver.info() != Success) abort();
+  cout << "The eigenvalues of A are:\n" << eigensolver.eigenvalues() << endl;
+  cout << "Here's a matrix whose columns are eigenvectors of A \n"
+       << "corresponding to these eigenvalues:\n"
+       << eigensolver.eigenvectors() << endl;
+}