Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/QuickStart_example2_fixed.cpp

@@ -0,0 +1,14 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+using namespace std;
+
+int main() {
+  Matrix3d m = Matrix3d::Random();
+  m = (m + Matrix3d::Constant(1.2)) * 50;
+  cout << "m =" << endl << m << endl;
+  Vector3d v(1, 2, 3);
+
+  cout << "m * v =" << endl << m * v << endl;
+}