Work in progress in the SpikeGenerator implementation. A remaining bug with the period argument needs to be fixed,
because not all the spikes are always replayed
Identification of the bug in the SpikeGenerator group: spikes in the last timesteps are not played
Fix the implementation of the SpikeGenerator with all templates, implementing the sliding offset and
the keyword period, to repeat sequences of spikes. Note that I had to change the fact that spikes
are now played when in between t and t+dt, and not as before, between t-dt and t. This is because,
otherwise, spikes in the last timestep were never played. This can be discussed