Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/special_examples/random_cpp11.cpp

@@ -0,0 +1,14 @@
+#include <Eigen/Core>
+#include <iostream>
+#include <random>
+
+using namespace Eigen;
+
+int main() {
+  std::default_random_engine generator;
+  std::poisson_distribution<int> distribution(4.1);
+  auto poisson = [&]() { return distribution(generator); };
+
+  RowVectorXi v = RowVectorXi::NullaryExpr(10, poisson);
+  std::cout << v << "\n";
+}