Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/ComplexEigenSolver_eigenvalues.cpp

@@ -0,0 +1,4 @@
+MatrixXcf ones = MatrixXcf::Ones(3, 3);
+ComplexEigenSolver<MatrixXcf> ces(ones, /* computeEigenvectors = */ false);
+cout << "The eigenvalues of the 3x3 matrix of ones are:" << endl
+     << ces.eigenvalues() << endl;