Correctly expose postsynaptic model extra global parameters to PyGeNN

[neworderofjamie]

Initially I thought more stuff could be broken so I added further tests but, turns out, there was simple a missing argument in the PyGeNN function to create custom postsynaptic models.

Fixes #514

[codecov]

Codecov Report

Merging #522 (5e3cf5f) into master (93a50ca) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
+ Coverage   86.96%   87.05%   +0.08%     
==========================================
  Files          84       84              
  Lines       18089    18090       +1     
==========================================
+ Hits        15731    15748      +17     
+ Misses       2358     2342      -16     

Impacted Files	Coverage Δ
include/genn/genn/postsynapticModels.h	100.00% <0.00%> (ø)
src/genn/genn/code_generator/groupMerged.cc	89.25% <0.00%> (+0.48%)	⬆️
src/genn/genn/code_generator/generateRunner.cc	96.53% <0.00%> (+0.53%)	⬆️
include/genn/genn/synapseGroup.h	92.45% <0.00%> (+1.88%)	⬆️
include/genn/genn/currentSource.h	88.88% <0.00%> (+11.11%)	⬆️

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[tnowotny]

Duh ... good catch.