Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/unsupported/doc/examples/MatrixSquareRoot.cpp

@@ -0,0 +1,15 @@
+#include <iostream>
+#include <unsupported/Eigen/MatrixFunctions>
+
+using namespace Eigen;
+
+int main() {
+  const double pi = std::acos(-1.0);
+
+  MatrixXd A(2, 2);
+  A << cos(pi / 3), -sin(pi / 3), sin(pi / 3), cos(pi / 3);
+  std::cout << "The matrix A is:\n" << A << "\n\n";
+  std::cout << "The matrix square root of A is:\n" << A.sqrt() << "\n\n";
+  std::cout << "The square of the last matrix is:\n"
+            << A.sqrt() * A.sqrt() << "\n";
+}