have macpan2 infer missing values in initial state vecs for internally calculated quantities?

[papsti]

from a draft of the quickstart vignette:

---

## SIR model simulator -------------------------
sir_simulator <- mp_tmb_simulator(
  dynamic_model = sir,
  vectors = list(
    state = c(S = 999, I = 1, R = 0),
    rate = c(beta = 0.25, gamma = 0.1, lambda = NA)
  ),
  time_steps = 100
)

Note that we've specified NA for lambda as it will be calculated for us using the force of infection functional form.

---

from @bbolker: is this required, or could macpan2 assume that unspecified values are set to NA automatically? (There could be an implicit/explicit design tradeoff here - i.e. it would be convenient to do this automatically but might be better to force users to be explicit ...)

[stevencarlislewalker]

Must also be robust to the order of values