Fix max connections issues

[neworderofjamie]

This fixes two issues:

1. Even if you've used a sparse connectivity initialisation snippet to initialise connectivity, as long as you're increasing it, you should still be able to manually increase max connections e.g. to leave a bit more padding at the end of each row for rewiring
2. Max connections wasn't being included in hashes used to determine if model had changed so if you had build a model based on some manually specified connectivity and changed it you ended up in a really broken state

[tnowotny]

Looks reasonable ...

[codecov]

Codecov Report

Merging #515 (17204e4) into master (f3c1a0f) will increase coverage by 2.68%.
The diff coverage is 23.07%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   86.81%   89.50%   +2.68%     
==========================================
  Files          84       73      -11     
  Lines       18063    10565    -7498     
==========================================
- Hits        15682     9456    -6226     
+ Misses       2381     1109    -1272     

Impacted Files	Coverage Δ
src/genn/genn/synapseGroup.cc	82.96% <16.66%> (-2.50%)	⬇️
src/genn/genn/code_generator/groupMerged.cc	86.05% <100.00%> (-2.72%)	⬇️
include/genn/genn/synapseGroupInternal.h	0.00% <0.00%> (-75.00%)	⬇️
src/genn/genn/code_generator/modelSpecMerged.cc	74.44% <0.00%> (-21.07%)	⬇️
include/genn/genn/snippet.h	74.35% <0.00%> (-10.72%)	⬇️
src/genn/genn/binomial.cc	63.79% <0.00%> (-5.89%)	⬇️
include/genn/genn/code_generator/codeStream.h	92.00% <0.00%> (-4.30%)	⬇️
src/genn/genn/code_generator/codeStream.cc	88.00% <0.00%> (-4.00%)	⬇️
include/genn/genn/code_generator/groupMerged.h	80.55% <0.00%> (-3.74%)	⬇️
src/genn/genn/currentSource.cc	78.43% <0.00%> (-3.39%)	⬇️
... and 63 more

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