Yin yang dataset, TTFS readout and event-prop implementation

[neworderofjamie]

• Added yin yang dataset generation based on https://github.com/lkriener/yin_yang_data_set as it's not packaged anywhere so annoying to include
• Added FirstSpikeTime readout which produces time of first spike for each neuron. Happy to debate this design decision but, this way, standard metrics and readouts can be applied.
• Added support for LIF output neurons with FirstSpikeTime readout to EventProp compiler
• Added EventProp example training on YinYang based on @tnowotny's https://github.com/tnowotny/genn_eventprop/blob/main/train_yinyang.py

Fixes #107

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 11.65049% with 182 lines in your changes missing coverage. Please review.

Project coverage is 64.18%. Comparing base (7bddc52) to head (98ffef0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
ml_genn/ml_genn/compilers/event_prop_compiler.py	3.37%	143 Missing ⚠️
ml_genn/ml_genn/utils/data.py	3.22%	30 Missing ⚠️
ml_genn/ml_genn/readouts/first_spike_time.py	50.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   65.22%   64.18%   -1.04%     
==========================================
  Files         101      102       +1     
  Lines        4218     4305      +87     
==========================================
+ Hits         2751     2763      +12     
- Misses       1467     1542      +75     

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