Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/CustomizingEigen_Inheritance.cpp

@@ -0,0 +1,27 @@
+#include <Eigen/Core>
+#include <iostream>
+
+class MyVectorType : public Eigen::VectorXd {
+ public:
+  MyVectorType(void) : Eigen::VectorXd() {}
+
+  // This constructor allows you to construct MyVectorType from Eigen
+  // expressions
+  template <typename OtherDerived>
+  MyVectorType(const Eigen::MatrixBase<OtherDerived>& other)
+      : Eigen::VectorXd(other) {}
+
+  // This method allows you to assign Eigen expressions to MyVectorType
+  template <typename OtherDerived>
+  MyVectorType& operator=(const Eigen::MatrixBase<OtherDerived>& other) {
+    this->Eigen::VectorXd::operator=(other);
+    return *this;
+  }
+};
+
+int main() {
+  MyVectorType v = MyVectorType::Ones(4);
+  v(2) += 10;
+  v = 2 * v;
+  std::cout << v.transpose() << std::endl;
+}