Check event code for den delay

[neworderofjamie]

Until Anindya tried, clearly no one has tried using heterogeneous delays and spike-like events - the event code was never getting searched for addToInSynDelay so code wasn't compiling. Also added a unit test

[codecov]

Codecov Report

Merging #494 (f3f87bc) into master (037af10) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #494   +/-   ##
=======================================
  Coverage   86.94%   86.94%           
=======================================
  Files          84       84           
  Lines       17972    17976    +4     
=======================================
+ Hits        15625    15629    +4     
  Misses       2347     2347           

Impacted Files	Coverage Δ
src/genn/genn/synapseGroup.cc	85.45% <100.00%> (+0.07%)	⬆️

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[tnowotny]

oh boy ... you do need users to find all the nasties ... looks like a straightforward fix, thanks.