flake8 on ensemble.
Provide global inhibition values to the SpiNNaker executable.

 - EnsembleVertex has reference to a inhibitory_edge
   - edge is registered by the builder - only one may exist
 - EnsembleVertex has new regions for filter data and routing data for
   the inhibitory connection
 - General filters will now check that the connection they represent do
   not correspond to a x -> Neurons connection.  Such connections are
   ignored when building the filter and filter routing entries.