Test single ODE versus two ODEs

[maxbiostat]

This analysis is to find out whether using the single-ODE parametrisation of the closed-population SIR leads to a substantial gain/loss in two respects:

(i) Computational: are there differences in running time?
(ii) Statistical: Do we get lower MCSE and higher ESS (n_eff)?

The single ODE implementation is here and the double ODE implementation is here, but we need to make sure EVERYTHING ELSE is the same between implementations to ensure comparability

[maxbiostat]

@marciomacielbastos , this is the task for this week.