Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

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

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