Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/PartialRedux_count.cpp

@@ -0,0 +1,6 @@
+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Matrix<ptrdiff_t, 3, 1> res = (m.array() >= 0.5).rowwise().count();
+cout << "Here is the count of elements larger or equal than 0.5 of each row:"
+     << endl;
+cout << res << endl;