Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/MatrixBase_diagonal_template_int.cpp

@@ -0,0 +1,7 @@
+Matrix4i m = Matrix4i::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here are the coefficients on the 1st super-diagonal and 2nd "
+        "sub-diagonal of m:"
+     << endl
+     << m.diagonal<1>().transpose() << endl
+     << m.diagonal<-2>().transpose() << endl;