Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

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

@@ -0,0 +1,15 @@
+#include <iostream>
+#include <unsupported/Eigen/MatrixFunctions>
+
+using namespace Eigen;
+
+int main() {
+  const double pi = std::acos(-1.0);
+  Matrix3d A;
+  A << cos(1), -sin(1), 0, sin(1), cos(1), 0, 0, 0, 1;
+  std::cout << "The matrix A is:\n"
+            << A << "\n\n"
+                    "The matrix power A^(pi/4) is:\n"
+            << A.pow(pi / 4) << std::endl;
+  return 0;
+}