[NixIO] Read and write channel_ids of ChannelIndex
[NixIO] Read and write channel_ids of ChannelIndex

Fixes #356

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