fix yticks in plot_spikes

[petermarathas]

This adds two y axis labels to the scatter plots in plot_spikes, corresponding to the beginning and end of the n_neuron range. It should fix an issue uncovered in #467 that plot_spikeswas not showing spikes for intermediate layers. The issue was that the y axis would have a small range around the spikes of the initial data entry, and this range would not cover the full span of possible neurons.

Before

After

[Hananel-Hazan]

Thanks @petermarathas