规范模块目录和第三方依赖结构

third_party/eigen3/doc/snippets/MatrixBase_eval.cpp

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+Matrix2f M = Matrix2f::Random();
+Matrix2f m;
+m = M;
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Now we want to copy a column into a row." << endl;
+cout << "If we do m.col(1) = m.row(0), then m becomes:" << endl;
+m.col(1) = m.row(0);
+cout << m << endl << "which is wrong!" << endl;
+cout << "Now let us instead do m.col(1) = m.row(0).eval(). Then m becomes" << endl;
+m = M;
+m.col(1) = m.row(0).eval();
+cout << m << endl << "which is right." << endl;