Allow multiple edges with the same STDP rule but different parameters

[rowleya]

Make it possible to have different parameters for the same STDP rule on multiple connections. This would require additional DTCM, but this would then be up to the user as to how much can be supported. This would also then work well with synaptic cores, since the different parameters could be split between the cores if needed (though this doesn't need to be a requirement).

[andrewgait]

By "different parameters", do you mean having (for example) a randomly distributed value for timing rule / weight rule parameters per synapse? Or values across different sets of synapses between the same two neuron populations?

[rowleya]

I think this was a one-per-edge deal. We get this in structural plasticity I believe, but not currently in STDP. We could also support a per-synapse thing if desired, although the size of the synapses would become large (but if that is what the users want, it can be done).