Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/QuickStart_example.cpp

@@ -0,0 +1,13 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using Eigen::MatrixXd;
+
+int main() {
+  MatrixXd m(2, 2);
+  m(0, 0) = 3;
+  m(1, 0) = 2.5;
+  m(0, 1) = -1;
+  m(1, 1) = m(1, 0) + m(0, 1);
+  std::cout << m << std::endl;
+}