enable passing of age-structured parameters in `params_timevar` argument for sims

[papsti]

Currently, the only way to vary parameters in time in the age-structured model is to scale an age-structure parameter vector (specified in the params list) by a single factor across all ages (specified in params_timevar). It would be useful for forecasting to be able to scale the age-structured param vector element-wise with a vector of factors specified in params_timevar (e.g. to simulate increases in transmission in alone kids by scaling the beta0 values for child age groups, leaving beta0 for adult age groups unchanged).

However, since params_timevar is specified as a data.frame (with one value per row), and run_sim() loops over the rows to do parameter updates, age-specific scaling factors can't currently be specified in params_timevar.

@bbolker suggested making use of unlist() to unpack age-specific parameters stored in a list called par (for instance), which will produce a named vector with names of the form par.i where i is an integer for i = 1, ..., n with n being the length of the par list. Then one could specify the Symbol in params_timevar as par.i to update parameter par for age group i. If we do this, the age-related stuff in run_sim() (and downstream) may have to be updated as currently, age-structured params are expected as vector in a params list entry (in the base model, params are just stored as a name vector since there is only one value per entry).