[NixIO] Split object map into two: Neo and NIX
[NixIO] Split object map into two: Neo and NIX

The IO would keep track of object mappings from Neo object ids to NIX
objects and vice versa. We now use the same key to track both kinds of
objects in two different maps: one for Neo and one for Neo and one for
NIX objects.

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