Add Diehl and Cook (2015) small model

[bjourne]

Hello Brian2 team,

Code below is my attempt at making a modest contribution to the Brian2 simulator. In Brian2's Discourse there has been some interest in this model so adding it as an example seems like a good idea. The code is a cleaned up and Python 3-compatible version of Diehl and Cook's original implementation and Bruce Tian's Brian2 implementation.

[mstimberg]

Many thanks! This model is discussed quite often on the forum, so adding it to the documentation would clearly make many users happy. I won't be able to have a closer look at your code before next week, but it is on my TODO list :-) Please do not hesitate to ping me if you don't hear anything from me…