[nixio] Test Segmentless objects
[nixio] Test Segmentless objects

Tests for objects existing in the ChannelIndex substructure without
being attached to a Segment.

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