Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/Tutorial_commainit_01b.cpp

@@ -0,0 +1,5 @@
+Matrix3f m;
+m.row(0) << 1, 2, 3;
+m.block(1, 0, 2, 2) << 4, 5, 7, 8;
+m.col(2).tail(2) << 6, 9;
+std::cout << m;