Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/SelfAdjointEigenSolver_operatorInverseSqrt.cpp

@@ -0,0 +1,13 @@
+MatrixXd X = MatrixXd::Random(4, 4);
+MatrixXd A = X * X.transpose();
+cout << "Here is a random positive-definite matrix, A:" << endl
+     << A << endl
+     << endl;
+
+SelfAdjointEigenSolver<MatrixXd> es(A);
+cout << "The inverse square root of A is: " << endl;
+cout << es.operatorInverseSqrt() << endl;
+cout
+    << "We can also compute it with operatorSqrt() and inverse(). That yields: "
+    << endl;
+cout << es.operatorSqrt().inverse() << endl;