Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/ComplexSchur_matrixT.cpp

@@ -0,0 +1,4 @@
+MatrixXcf A = MatrixXcf::Random(4, 4);
+cout << "Here is a random 4x4 matrix, A:" << endl << A << endl << endl;
+ComplexSchur<MatrixXcf> schurOfA(A, false);  // false means do not compute U
+cout << "The triangular matrix T is:" << endl << schurOfA.matrixT() << endl;