Boosting

[giovannic]

Contingent on #21

Todo:

• [ ] Update vaccine strategy parameters to take a boosting times
• [ ] Update the vaccine distribution process to distribute at boost times as well

Boosting times parameter is just a vector of timesteps after initial vaccination when the vaccination will be redone (as though it's the first time)

[pwinskill]

Boosting can be defined as addtional dose(s) of the same vaccine to an individual who has already recieved the initial full schedule of that vaccine. This may be important if vaccine-derived protection is not long-lasting and regular boosters are required.

I think to specify this we just need to track which vaccines an individual has had in the past and the time points of previous doses (possibly just the last dose). We would then also need to parameterise the magnitude in the boost to immunity provided by booster dose at a given timepoint after the last dose.

A resasonable first approach may be to model the booster as producing the same immune response as the intial schedule of doses - essentially just "updating".