Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/Tridiagonalization_packedMatrix.cpp

@@ -0,0 +1,9 @@
+Matrix4d X = Matrix4d::Random(4, 4);
+Matrix4d A = X + X.transpose();
+cout << "Here is a random symmetric 4x4 matrix:" << endl << A << endl;
+Tridiagonalization<Matrix4d> triOfA(A);
+Matrix4d pm = triOfA.packedMatrix();
+cout << "The packed matrix M is:" << endl << pm << endl;
+cout << "The diagonal and subdiagonal corresponds to the matrix T, which is:"
+     << endl
+     << triOfA.matrixT() << endl;