Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/Tutorial_PartialLU_solve.cpp

@@ -0,0 +1,17 @@
+#include <Eigen/Core>
+#include <Eigen/LU>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  Matrix3f A;
+  Vector3f b;
+  A << 1, 2, 3, 4, 5, 6, 7, 8, 10;
+  b << 3, 3, 4;
+  cout << "Here is the matrix A:" << endl << A << endl;
+  cout << "Here is the vector b:" << endl << b << endl;
+  Vector3f x = A.lu().solve(b);
+  cout << "The solution is:" << endl << x << endl;
+}