[nixio] Major rewrite: ChannelIndex reading
[nixio] Major rewrite: ChannelIndex reading

A reference map is populated while reading signal objects in order to
create the references between ChannelIndex and signals.

INM-6/python-neo,NeuralEnsemble/python-neo,samuelgarcia/python-neo,apdavison/python-neo,rgerkin/python-neo,JuliaSprenger/python-neo