Complete LU documentation

doc/snippets/class_LU_1.cpp

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+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Eigen::LU<Matrix3d> lu(m);
+cout << "Here is, up to permutations, its LU decomposition matrix:"
+     << endl << lu.matrixLU() << endl;
+cout << "Let us now reconstruct the original matrix m from it:" << endl;
+Matrix3d x = lu.matrixL() * lu.matrixU();
+Matrix3d y;
+for(int i = 0; i < 3; i++) for(int j = 0; j < 3; j++)
+  y(i, lu.permutationQ()[j]) = x(lu.permutationP()[i], j);
+cout << y << endl;
+assert(y.isApprox(m));