into-ns: create follow-up question to 3. part of diffreaction lecture

[lindaudby]

In diffraction topic of into-ns: create follow-up question to 3. part of diffraction video lecture
Question: what is the value of the lattice sum?
Right answer: $N \frac{(2\pi)^3}{V_0}\sum{\tau}\delta{\mathbf{q}-\mathbf{tau}}$.
We will collect other answers form Ku2017 course to implement as multiple choice.

We need to fix latex in these "choices questions" in moodle before implementation ...