Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/tut_arithmetic_matrix_mul.cpp

@@ -0,0 +1,17 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+int main() {
+  Matrix2d mat;
+  mat << 1, 2, 3, 4;
+  Vector2d u(-1, 1), v(2, 0);
+  std::cout << "Here is mat*mat:\n" << mat * mat << std::endl;
+  std::cout << "Here is mat*u:\n" << mat * u << std::endl;
+  std::cout << "Here is u^T*mat:\n" << u.transpose() * mat << std::endl;
+  std::cout << "Here is u^T*v:\n" << u.transpose() * v << std::endl;
+  std::cout << "Here is u*v^T:\n" << u * v.transpose() << std::endl;
+  std::cout << "Let's multiply mat by itself" << std::endl;
+  mat = mat * mat;
+  std::cout << "Now mat is mat:\n" << mat << std::endl;
+}