Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/MatrixBase_template_int_start.cpp

@@ -0,0 +1,5 @@
+RowVector4i v = RowVector4i::Random();
+cout << "Here is the vector v:" << endl << v << endl;
+cout << "Here is v.head(2):" << endl << v.head<2>() << endl;
+v.head<2>().setZero();
+cout << "Now the vector v is:" << endl << v << endl;