Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/TopicAliasing_mult2.cpp

@@ -0,0 +1,10 @@
+MatrixXf matA(2, 2), matB(2, 2);
+matA << 2, 0, 0, 2;
+
+// Simple but not quite as efficient
+matB = matA * matA;
+cout << matB << endl << endl;
+
+// More complicated but also more efficient
+matB.noalias() = matA * matA;
+cout << matB;