Custom update merge fix

[neworderofjamie]

In the course of my structural plasticity implementing, I found not one but two bugs in the variable reference/custom update stuff!

Merging

In general, when merging together custom updates, what the variable references target doesn't really matter - they're just pointers. However, whether variable references point to variables with VarAccessDuplication::DUPLICATE, VarAccessDuplication::SHARED etc changes the required index calculating code so this needs to be considered. This PR includes the variable duplication mode in the hashes used for merging groups and extends the unit tests to better cover corner cases in this area.

References to custom update variables

Unlike synapses and neurons which are always batched if the model is, if custom updates are batched or not depends on whether they are attached to VarAccessDuplication::DUPLICATE variables (and whether the model is batched). This means if you make references to these variables from other custom updates whether these 'downstream' custom updates should be batched should depend on whether the upstream custom update was batched as well as the mode of the variables. This PR adds a isBatched method to each variable reference and extends the unit tests to cover this.

[codecov]

Codecov Report

Base: 88.80% // Head: 88.81% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (58a8801) compared to base (0312dc5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #557   +/-   ##
=======================================
  Coverage   88.80%   88.81%           
=======================================
  Files          76       76           
  Lines       12344    12354   +10     
=======================================
+ Hits        10962    10972   +10     
  Misses       1382     1382           

Impacted Files	Coverage Δ
include/genn/genn/customUpdate.h	95.31% <100.00%> (+0.15%)	⬆️
include/genn/genn/models.h	66.66% <100.00%> (+1.44%)	⬆️
src/genn/genn/customUpdate.cc	93.65% <100.00%> (+0.10%)	⬆️
src/genn/genn/models.cc	96.39% <100.00%> (+0.16%)	⬆️

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[tnowotny]

Wow, it does get complicated with merging/ hashing/ custom updates ... but it looks like a sensible solution.

[neworderofjamie]

Yeah - I feel maybe there might be some extra constraints that could be added which would simplify the whole system somewhat e.g. I'm pretty sure I haven't yet had a use-case for a custom update that simultaneously references variables in different populations of the size (although this is supported)