Make SpikeGeneratorGroup work for run_args

[mstimberg]

The run_args feature is very useful for simulations where you want to present different stimuli to a network, but currently this only works if you can represent the stimulus as the initial value of a state variable (e.g. the rates of a PoissonGroup), or as the values in a TimedArray. You cannot change the values of a SpikeGeneratorGroup, since it currently checks and prepares the data structures in Python code. If we moved this preparation into the templates, it would become possible to switch out the spikes indices/times for every run via run_args. See this related discussion: https://brian.discourse.group/t/dynamically-loading-spiketimes-in-a-spikegeneratorgroup-on-c-standalone-mode/1369/3