support specification of population size by age

[papsti]

a user can now pass a new population vector (count by age) into the values fields of either make_simulator() or simulate() (in a named list under the entry "pop"). (value updates through simulate() is still only supported for the hosp model)

note that if the population sizes are being updated, the contact parameters must also be regenerated to ensure the average contact rate by age (c.hat) remains consistent with the new population data.

this update can be done with

values$pop <- c(...) # new population size vector
values$contact.pars <- mk_contact_pars(
    age.group.lower = seq(0, 80, by = 5),
    setting.weight = values$setting.weight, 
    pop.new = values$pop
)