[nixio] Major rewrite: Append existing multiref objects
[nixio] Major rewrite: Append existing multiref objects

When the same data object (Signal, Event, Epoch, or SpikeTrain) is
attached to multiple Segments, the IO should not attempt to recreate it,
but instead take it from the root Block and append it to the Group.

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