pep 8
Progress towards using MPI.

* chunk.py contains a python-side representation of the lower level MPI
  chunks. Each of these chunks represents what a single MPI process will
  simulate

* partitioning now occurs on the python side. a simulator chunk is
  created for each partition, and the correct operators and signals are
  added to each chunk.

* NEF connections that connect NEF ensembles that are part of different
  partitions get split at the decoded signal. That is, the decoded
  signal is what gets communicated by MPI

* MPI operators get added by each simulator chunk in the correct places.