规范模块目录和第三方依赖结构

third_party/eigen3/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;