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 19.44444% with 87 lines in your changes missing coverage. Please review.

Project coverage is 63.75%. Comparing base (af2d5da) to head (435c7a8).

Files with missing lines	Patch %	Lines
ml_genn/ml_genn/compilers/event_prop_compiler.py	4.34%	44 Missing :warning:
ml_genn/ml_genn/utils/data.py	2.85%	34 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 #117 +/- ## ========================================== - Coverage 64.80% 63.75% -1.06% ========================================== Files 101 102 +1 Lines 4259 4345 +86 ========================================== + Hits 2760 2770 +10 - Misses 1499 1575 +76 ```

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