Move functionality of SynapseView to synapse_utils

[jnsbck]

Before #447 synapses could be "viewed" using SynapseView, i.e. using net.TestSynapse. This functionality is retained, but net.TestSynapse([0,1]) was replaced by net.TestSynapse.edge([0,1]). After discussion with @kyralianaka and @michaeldeistler we decided to remove edge, net.TestSynapse and also the channel views added in #447 (net.HH). This functionality should be moved to synapse_utils and can make heavy use net.select.

white board sketches made during the discussion