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 :warning:
ml_genn/ml_genn/utils/data.py	3.22%	30 Missing :warning:
ml_genn/ml_genn/readouts/first_spike_time.py	50.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ 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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