Batched prev spike time

[neworderofjamie]

Two fixes here:

1. Missing numNeurons field in NeuronPrevSpikeTimeUpdateGroupMerged required when model has batch size > 1. Not 100% sure why batch size isn't readily available in this constructor but unconditionally including the number of neurons is fine
2. Indexing when updating previous spike times was incorrect when model is batched but there are no delays (both group->prevST and group->spk are num_batches * num_neurons size arrays so indices into both need to have batchOffset adding to them)

I have also updated the feature test which nearly covered this to actually do so!

Fixes #564

[codecov]

Codecov Report

Base: 88.80% // Head: 89.31% // Increases project coverage by +0.51% 🎉

Coverage data is based on head (1063c0a) compared to base (dc50d25).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   88.80%   89.31%   +0.51%     
==========================================
  Files          76       76              
  Lines       12358    12359       +1     
==========================================
+ Hits        10975    11039      +64     
+ Misses       1383     1320      -63     

Impacted Files	Coverage Δ
src/genn/genn/code_generator/backendSIMT.cc	94.79% <40.00%> (+0.94%)	⬆️
src/genn/genn/code_generator/groupMerged.cc	86.94% <100.00%> (-0.02%)	⬇️
src/genn/backends/opencl/backend.cc	92.45% <0.00%> (+0.99%)	⬆️
include/genn/genn/code_generator/backendBase.h	88.00% <0.00%> (+1.33%)	⬆️
src/genn/backends/cuda/optimiser.cc	81.51% <0.00%> (+10.30%)	⬆️
src/genn/backends/opencl/optimiser.cc	95.45% <0.00%> (+18.18%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[tnowotny]

Yes, that looks like I would have expected. Mysterious that testing for batchsize > 1 is so elusive in the definition of the update group but indeed not much harm done to include numNeurons always.
Also Fabian has confirmed his code now works as he expected.