* Consistent use of globally defined integer types for
  indices etc. (catypes.hpp) or template type parameters
  across simulator.
* Support unsigned types in tree, cell_tree and friends.
* Use cell_member_type for probe ids (WIP)
* Rejig mechanism lookup structure (simplifies, plays nicely
  with templated index types.)
* Move spike, connection, events out of communcation namespace
  and directory.