Postsynaptic model target

[neworderofjamie]

As described in #451, previously, you needed to define a new postsynaptic model for each additional input variable you might want to connect synapses to e.g. 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;
    """)

just to connect synapses to a neuron's ISynFeedback additional input variable. This PR instead allows you to do this in C++:

auto *syn= model.addSynapsePopulation<WeightUpdateModels::StaticPulse, PostsynapticModels::DeltaCurr>(
    "Syn", SynapseMatrixType::DENSE_INDIVIDUALG, NO_DELAY,
    "Pre", "Post",
    {}, { 1.0 },
    {}, {});
syn->setPSOutputVar("ISynFeedback");

or in Python

syn = model.add_synapse_population("Syn", "DENSE_INDIVIDUALG", NO_DELAY,
    pre_pop, post_pop,
    "StaticPulse", {}, {"g": 1.0}, {}, {},
    "DeltaCurr", {}, {})
syn.ps_output_var = "ISynFeedback"

The implementation is trivial, my only question is whether "PS output variable" is a good names for these things rather than, for instance, "Target input variable" or "Postsynaptic input variable". Any thoughts much appreciated!

Fixes #451

[codecov]

Codecov Report

Merging #458 (2a6d482) into master (967ffba) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   87.21%   87.23%   +0.01%     
==========================================
  Files          78       78              
  Lines       17038    17057      +19     
==========================================
+ Hits        14860    14879      +19     
  Misses       2178     2178              

Impacted Files	Coverage Δ
include/genn/genn/synapseGroup.h	90.90% <100.00%> (+0.21%)	⬆️
...c/genn/genn/code_generator/generateNeuronUpdate.cc	95.66% <100.00%> (+0.02%)	⬆️
src/genn/genn/synapseGroup.cc	81.88% <100.00%> (+0.49%)	⬆️

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[tnowotny]

Yes, all good but could consider a different name. I think input/output maybe not ideal concepts as one's input is the other one's output.
How about "PSTargetVar"?
I think you have used "PS" for post-synaptic before so is in good keeping with that practice.

[neworderofjamie]

Excellent suggestion!