Neaten the simulator.
Neaten the simulator.

 - Raise a NotImplementedException if `run` is called twice without an
   intervening `reset`.
 - Move a lot of the PACMAN code out of `run` into a private method.
 - Provide the `reset` method
 - Add a lot more comments

ctn-archive/nengo_spinnaker_2014,ctn-archive/nengo_spinnaker_2014,ctn-archive/nengo_spinnaker_2014,ctn-archive/nengo_spinnaker_2014