Extra Global Parameter references

[neworderofjamie]

My per-timestep EventProp loss implementation required extra global parameters to be shared between a neuron group and a custom update which, as we have talked for a while, is not an uncommon requirement. However, all my attempts to design some sort of extra global parameter sharing system ended up looking very messy so, instead, here I added the ability to reference extra global parameters in custom updates. This is:

• Simple - the existing system for updating pointers in merged groups can handle it without modification so it's totally transparent to the lower levels of GeNN
• Familiar - the syntax is almost identical to variable references so
• Safer - by forcing custom updates to be involved, it discourages users from trying to use these to connect neuron groups together in weird ways that will probably fail silently and not-deterministically

A model with EGP refs might look like:

custom_update_model = genn_model.create_custom_custom_update_class(
    "custom_update",
    var_refs=[("x", "scalar")],
    extra_global_param_regs=[("e", "scalar*")],
    update_code="""
    if($(id) == (int)round(fmod($(t), 10.0))) {
        $(e)[$(id)] = 1.0;
    }
    else {
        $(e)[$(id)] = 0.0;
    }
    """)

Then you might add it to a model and configure the EGP reference like:

model.add_custom_update("CustomUpdate1", "Group", custom_update_model,
                        {}, {}, {"x": genn_model.create_var_ref(pop, "V"}, 
                        {"e": genn_model.create_egp_ref(pop, "E")})

where pop is some sort of population with a V state variable and an EGP called E. Note that, in this useless example, the x var reference is totally unused - it's just there to define the 'size' of the custom update.

[codecov]

Codecov Report

Patch coverage: 77.77% and project coverage change: -0.06 ⚠️

Comparison is base (ef69e59) 88.79% compared to head (1648e37) 88.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #583      +/-   ##
==========================================
- Coverage   88.79%   88.74%   -0.06%     
==========================================
  Files          76       76              
  Lines       12428    12490      +62     
==========================================
+ Hits        11036    11084      +48     
- Misses       1392     1406      +14     

Impacted Files	Coverage Δ
include/genn/genn/customUpdateInternal.h	0.00% <ø> (ø)
include/genn/genn/modelSpec.h	100.00% <ø> (ø)
src/genn/genn/models.cc	88.32% <53.84%> (-8.08%)	⬇️
include/genn/genn/models.h	68.75% <75.00%> (+2.08%)	⬆️
src/genn/genn/customUpdateModels.cc	93.75% <83.33%> (-6.25%)	⬇️
...enn/genn/code_generator/customUpdateGroupMerged.cc	90.14% <85.71%> (-0.16%)	⬇️
include/genn/genn/code_generator/groupMerged.h	83.18% <100.00%> (+0.35%)	⬆️
include/genn/genn/code_generator/modelSpecMerged.h	99.19% <100.00%> (ø)
include/genn/genn/customUpdate.h	94.91% <100.00%> (-0.47%)	⬇️
src/genn/genn/customUpdate.cc	94.96% <100.00%> (+0.48%)	⬆️

... and 1 file with indirect coverage changes

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[tnowotny]

Seems a straightforward addition