Brought up to date with nengo.

Nearing a nengo release, so upgraded nengo_mpi to work with that version
of nengo (assuming it won't change much between now and the release).

Most significant changes:

* Removal of ProdUpdate operator.

* Addition of ElementwiseInc operator.

* All signals on C++ side are now ublas Matrices whereas Vectors were
  being used before in many places. This was to make it easier to
  implement the ElementwiseInc operator, and makes things more
  consistent in general, requiring less special case code. The only
  negative part is slightly more verbose indexing for things that we
  know to be vectors, but we might be able to fix this later using
  matrix proxies.

* Now split up the nengo graph at the output of filters/synapses instead
  of at the output of decoding. This is to be consistent with a change
  in nengo which made the decoded process a non-update operation. Now
  the only source of necessary updates/delays in nengo is the output of
  filters, so it makes the most sense to break up the graph there.