Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/class_FixedReshaped.cpp

@@ -0,0 +1,18 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+template <typename Derived>
+Eigen::Reshaped<Derived, 4, 2> reshape_helper(MatrixBase<Derived>& m) {
+  return Eigen::Reshaped<Derived, 4, 2>(m.derived());
+}
+
+int main(int, char**) {
+  MatrixXd m(2, 4);
+  m << 1, 2, 3, 4, 5, 6, 7, 8;
+  MatrixXd n = reshape_helper(m);
+  cout << "matrix m is:" << endl << m << endl;
+  cout << "matrix n is:" << endl << n << endl;
+  return 0;
+}