Added repressilator model

[MichaelClerx]

See #227

[codecov]

Codecov Report

Merging #368 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
+ Coverage   85.57%   85.74%   +0.17%     
==========================================
  Files          39       40       +1     
  Lines        3071     3108      +37     
==========================================
+ Hits         2628     2665      +37     
  Misses        443      443

Impacted Files	Coverage Δ
pints/toy/__init__.py	100% <100%> (ø)	⬆️
pints/toy/_repressilator_model.py	100% <100%> (ø)

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