Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/TutorialLinAlgSetThreshold.cpp

@@ -0,0 +1,15 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  Matrix2d A;
+  A << 2, 1, 2, 0.9999999999;
+  FullPivLU<Matrix2d> lu(A);
+  cout << "By default, the rank of A is found to be " << lu.rank() << endl;
+  lu.setThreshold(1e-5);
+  cout << "With threshold 1e-5, the rank of A is found to be " << lu.rank()
+       << endl;
+}