Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/function_taking_ref.cpp

@@ -0,0 +1,18 @@
+#include <Eigen/SVD>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+float inv_cond(const Ref<const MatrixXf>& a) {
+  const VectorXf sing_vals = a.jacobiSvd().singularValues();
+  return sing_vals(sing_vals.size() - 1) / sing_vals(0);
+}
+
+int main() {
+  Matrix4f m = Matrix4f::Random();
+  cout << "matrix m:" << endl << m << endl << endl;
+  cout << "inv_cond(m):          " << inv_cond(m) << endl;
+  cout << "inv_cond(m(1:3,1:3)): " << inv_cond(m.topLeftCorner(3, 3)) << endl;
+  cout << "inv_cond(m+I):        " << inv_cond(m + Matrix4f::Identity())
+       << endl;
+}