stochasticity within a level of parameter set

[levisc8]

Given multiple parameter sets, allow for stochastic models to be stochastic within a level of one or more (e.g. stochastic across years within sites).

Interface:

par_set_list <- list(yr = 2005:2012, site = c("A", "B", "C")

define_kernel(..., uses_par_sets = TRUE, par_set_indices = par_set_list) %>%
define_impl(..., stoch_within = "site")

^^ Should produce 3 separate stochastic models for sites A,B,C using years 2005:2012 at each one.

NB: Not sure if this should go within define_impl or somewhere else - maybe define_pop_state() or make_ipm()