Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/LeastSquaresNormalEquations.cpp

@@ -0,0 +1,4 @@
+MatrixXf A = MatrixXf::Random(3, 2);
+VectorXf b = VectorXf::Random(3);
+cout << "The solution using normal equations is:\n"
+     << (A.transpose() * A).ldlt().solve(A.transpose() * b) << endl;