Smarter fusing of pre and postsynaptic models

[neworderofjamie]

#461 fused pre and postsynaptic weight update model code in the neuron kernel if the whole weight update model is identical but, in some models you have multiple learning rules which have the same pre or postsynaptic update e.g. various rules used in eProp models have this presynaptic update:

pre_var_name_types=[("ZFilter", "scalar")],
pre_spike_code="""
$(ZFilter) += 1.0;
""",
pre_dynamics_code="""
$(ZFilter) *= $(Alpha);
""",

But, currently, won't be fused as other parts of the model differ.