Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/MatrixBase_noalias.cpp

@@ -0,0 +1,5 @@
+Matrix2d a, b, c;
+a << 1, 2, 3, 4;
+b << 5, 6, 7, 8;
+c.noalias() = a * b;  // this computes the product directly to c
+cout << c << endl;