Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/SelfAdjointEigenSolver_operatorSqrt.cpp

@@ -0,0 +1,10 @@
+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);
+MatrixXd sqrtA = es.operatorSqrt();
+cout << "The square root of A is: " << endl << sqrtA << endl;
+cout << "If we square this, we get: " << endl << sqrtA * sqrtA << endl;