规范模块目录和第三方依赖结构

third_party/eigen3/doc/snippets/TopicAliasing_block.cpp

@@ -0,0 +1,7 @@
+MatrixXi mat(3,3); 
+mat << 1, 2, 3,   4, 5, 6,   7, 8, 9;
+cout << "Here is the matrix mat:\n" << mat << endl;
+
+// This assignment shows the aliasing problem
+mat.bottomRightCorner(2,2) = mat.topLeftCorner(2,2);
+cout << "After the assignment, mat = \n" << mat << endl;