Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/unsupported/doc/examples/PolynomialUtils1.cpp

@@ -0,0 +1,23 @@
+#include <iostream>
+#include <unsupported/Eigen/Polynomials>
+
+using namespace Eigen;
+using namespace std;
+
+int main() {
+  Vector4d roots = Vector4d::Random();
+  cout << "Roots: " << roots.transpose() << endl;
+  Eigen::Matrix<double, 5, 1> polynomial;
+  roots_to_monicPolynomial(roots, polynomial);
+  cout << "Polynomial: ";
+  for (int i = 0; i < 4; ++i) {
+    cout << polynomial[i] << ".x^" << i << "+ ";
+  }
+  cout << polynomial[4] << ".x^4" << endl;
+  Vector4d evaluation;
+  for (int i = 0; i < 4; ++i) {
+    evaluation[i] = poly_eval(polynomial, roots[i]);
+  }
+  cout << "Evaluation of the polynomial at the roots: "
+       << evaluation.transpose();
+}