This function compute the daily local FoI as a function
of the daily EIR and effects of partial immunity.
It is computed based on a probabilistic model of exposure and
possibly including environmental heterogeneity. If a model of human / host
infection has terms that describe partial immunity, e.g. affecting
pre-erythrocytic immunity to malaria called by F_b, those effects are implemented here.
The method dispatches on the type of pars$FOIpar.
F_foi(eir, b, pars)the daily eir for each stratum
the probability of infection, per bite
the model object as a list
the daily, local force of infection as a numeric vector