Improved corpora_management handling of names

Now if create_namedict is supplied to VectorizedCorpus.__init__,
then name2key and key2name dictionaries will be created as members.
In find_chain, can restrict keys to start at.
Brought neural extraction up to date.

Recent changes to nengo syntax, in particular the names of the
arguments to EnsembleArray and CircularConvolution. Also got rid of all
the cruft with the decoder solvers.
hierarchical simulation plot now works.