Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

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

@@ -0,0 +1,17 @@
+#include <iostream>
+#include <unsupported/Eigen/MatrixFunctions>
+
+using namespace Eigen;
+
+std::complex<double> expfn(std::complex<double> x, int) { return std::exp(x); }
+
+int main() {
+  const double pi = std::acos(-1.0);
+
+  MatrixXd A(3, 3);
+  A << 0, -pi / 4, 0, pi / 4, 0, 0, 0, 0, 0;
+
+  std::cout << "The matrix A is:\n" << A << "\n\n";
+  std::cout << "The matrix exponential of A is:\n"
+            << A.matrixFunction(expfn) << "\n\n";
+}