Initial stab at probing Nodes, PassNodes.
Ignore Synapses on PassNode probes where the inputs to the PassNode have synapses.

- `utils.probes.get_corrected_probes` provides warnings and modifies
  probes.
Merge branch 'master' into probe_nodes

Conflicts:
	nengo_spinnaker/value_sink_vertex.py
Correct merge.
Merge pull request #58 from ctn-waterloo/probe_nodes

Initial stab at probing Nodes, PassNodes.