Value-based tests for toy models

[ben18785]

• [x] Constant
• [x] Beeler-Reuter
• [x] Fitzhugh-Nagumo
• [x] Goodwin Oscillator
• [x] HES1 Michaelis-Menten
• [x] HH IK
• [x] Logistic model
• [x] Lotka-Volterra
• [x] Repressilator
• [x] SIR model

[MichaelClerx]

For what it's worth I've just checked both Fitzhugh-Nagumo and the Logistic model against an external implementation (sensitivities included) :D