Recording of a population should not record the population twice when population.record() is called twice, i.e. by accident

[JensKremkow]

I recorded my whole network twice, because I stupidly run my recording part of my network twice. PyNN connected thereby the same spike_detector twice to a neuron, which then printed each spikes twice, which gave my some hard time. PyNN should maybe check if a neuron is already recorded before recording it twice.

[apdavison]

Fixed in r259