Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/Tutorial_AdvancedInitialization_Zero.cpp

@@ -0,0 +1,11 @@
+std::cout << "A fixed-size array:\n";
+Array33f a1 = Array33f::Zero();
+std::cout << a1 << "\n\n";
+
+std::cout << "A one-dimensional dynamic-size array:\n";
+ArrayXf a2 = ArrayXf::Zero(3);
+std::cout << a2 << "\n\n";
+
+std::cout << "A two-dimensional dynamic-size array:\n";
+ArrayXXf a3 = ArrayXXf::Zero(3, 4);
+std::cout << a3 << "\n";