Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/TopicAliasing_block.cpp

@@ -0,0 +1,7 @@
+MatrixXi mat(3, 3);
+mat << 1, 2, 3, 4, 5, 6, 7, 8, 9;
+cout << "Here is the matrix mat:\n" << mat << endl;
+
+// This assignment shows the aliasing problem
+mat.bottomRightCorner(2, 2) = mat.topLeftCorner(2, 2);
+cout << "After the assignment, mat = \n" << mat << endl;