Increased max delay

[neworderofjamie]

Heterogeneous delays were previously always stored in uint8_t, limiting them to a maximum of 255 timesteps. @mbalazs98 code this is based on always switched to uint16_t which is wasteful.

This PR implements automatic switching to uint16_t if maximum delay is >= 256

Fixes #113

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.50000% with 30 lines in your changes missing coverage. Please review.

Project coverage is 63.62%. Comparing base (493d598) to head (e8b318e).

Files with missing lines	Patch %	Lines
ml_genn/ml_genn/compilers/compiler.py	21.05%	15 Missing ⚠️
ml_genn/ml_genn/compilers/event_prop_compiler.py	48.00%	13 Missing ⚠️
ml_genn/ml_genn/compilers/weight_update_models.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   63.70%   63.62%   -0.09%     
==========================================
  Files         102      102              
  Lines        4348     4360      +12     
==========================================
+ Hits         2770     2774       +4     
- Misses       1578     1586       +8     

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