Make pointing synapse groups at additional input variables easier

[neworderofjamie]

Currently you need to define a new postsynaptic model for each additional input variable i.e. you should be able to use a standard "DeltaCurr" rather than having to define:

feedback_psm_model = genn_model.create_custom_postsynaptic_class(
    "feedback_psm",
    apply_input_code="""
    $(ISynFeedback) += $(inSyn);
    $(inSyn) = 0;
    """)

It would be much nicer if the output variable of the apply input code was just a symbol and you could just call a method on the synapse group to set which variable it targets.