add in individual-level variation in infectiousness

[cwhittaker1000]

Currently all individuals implicitly have the same infectiousness in our model (i.e. the same β) - we should add some individual-level variation on top of this.

Anticipate this will look something like a draw from a lognormal distribution with a mean centred around 1.
rlnorm(n, meanlog = 0, sdlog = 1)
Log-normal selected as it approximately preserves the amount of mass either side of 1 on the relative scale (e.g. p(X<0.5) = p(X>2). Open to other suggestions though.

[cwhittaker1000]

Changing priority level of this to reflect advancements in our understanding.