Complete LU documentation

doc/snippets/LU_kernel.cpp

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+MatrixXf m = MatrixXf::Random(3,5);
+cout << "Here is the matrix m:" << endl << m << endl;
+MatrixXf ker = m.lu().kernel();
+cout << "Here is a matrix whose columns form a basis of the kernel of m:"
+     << endl << ker << endl;
+cout << "By definition of the kernel, m*ker is zero:"
+     << endl << m*ker << endl;