Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

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

@@ -0,0 +1,13 @@
+#include <iostream>
+#include <unsupported/Eigen/MatrixFunctions>
+
+using namespace Eigen;
+
+int main() {
+  using std::sqrt;
+  MatrixXd A(3, 3);
+  A << 0.5 * sqrt(2), -0.5 * sqrt(2), 0, 0.5 * sqrt(2), 0.5 * sqrt(2), 0, 0, 0,
+      1;
+  std::cout << "The matrix A is:\n" << A << "\n\n";
+  std::cout << "The matrix logarithm of A is:\n" << A.log() << "\n";
+}