Allow vertices to define a `prepare_vertex` function which will be called just once at some point in the build process.
Allow vertices to define a `prepare_vertex` function which will be called just once at some point in the build process.

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