Build decoders once and transform as necessary.
Compress decoders, get valid dimensions.

Construct decoders where all zero columns have been removed.
Perform this action on groups of decoders and tuples representing the
"header", index and dimension of each remaining dimension in the block.
"Headers" are for the purpose of attaching KeySpace generators or
similar.
EnsembleVertex uses new decoder tools.
Correct test.
Merge branch 'master' into decoders

Conflicts:
	nengo_spinnaker/utils/__init__.py
Merge branch 'master' into decoders
Fix for decoded calculation bug.
Fix for decoder solve fix.
Merge branch 'master' into decoders
Merge branch 'master' into decoders
Add a threshold to the decoder compression.
Merge pull request #61 from ctn-waterloo/decoders

Decoder Neatening