Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

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

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <unsupported/Eigen/MatrixFunctions>
+
+using namespace Eigen;
+
+int main() {
+  MatrixXd A = MatrixXd::Random(3, 3);
+  std::cout << "A = \n" << A << "\n\n";
+
+  MatrixXd sinA = A.sin();
+  std::cout << "sin(A) = \n" << sinA << "\n\n";
+
+  MatrixXd cosA = A.cos();
+  std::cout << "cos(A) = \n" << cosA << "\n\n";
+
+  // The matrix functions satisfy sin^2(A) + cos^2(A) = I,
+  // like the scalar functions.
+  std::cout << "sin^2(A) + cos^2(A) = \n"
+            << sinA * sinA + cosA * cosA << "\n\n";
+}