periodic models

[levisc8]

Current interface assumes all kernels operate on the same time scale without intermediate states. Need an interface for sequential kernels acting on intermediate states - think it should look something like this:

init_ipm(..., is_periodic = TRUE) %>%
... %>%
define_impl(
  state_start = c('size', 'size_1_star', 'size_2_star`),
  state_end  = c('size_1_star', 'size_2_star', 'size')
) %>%
define_pop_state(
  ...
  period_seq = c('size', 'size_1_star', 'size_2_star', 'size')
)