[nixrawio] Read ChannelIndex annotations from Source
[nixrawio] Read ChannelIndex annotations from Source

Was populating the ChannelIndex annotations from the Signal (DataArray)
metadata instead.

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