Update programming.tex

Including Michael's commentary that the "Matt trick" only helps hierarchical models when there is a lot of data. See <https://groups.google.com/d/msg/stan-users/dFFTF0n6mYo/WN3vWOf9ktAJ> and <https://groups.google.com/d/msg/stan-users/dFFTF0n6mYo/lTIfqVN8AB8J>.