"one step ahead" predictions to assess the fit to data

[lentinj]

Support the use of TMB::oneStepPredict() in gadget3-generated models. This
will require the addition of an “indicator variable”, with the same dimensions as the
observations array, used to turn individual observations on/off when calculating
likelihood. Add examples to the demo-ling model to show how it could be used.

[lentinj]

It's not the same dimensions as the observations array, it's the same dimensions of nllstock, i.e. if broken down by model timestep a on/off switch at each timestep.

This means all previous concerns about on/offing nonsense dimensions go out the window. nllstock is at most a vector over time.

This is nonsense. Then it doesn't match the observations array, which is the whole point.

[lentinj]

The main decision left here is how it's off-on'ed. We can just add it everywhere but doing so creates a useless array of 1's.

However, adding explicit "indicator_var" booleans to g3l_distribution_sumofsquares and friends would be a bit annoying to (forget to) off/on when required.

[lentinj]

An attempt at doing this is in the commit above. Parking this for now until we've worked out how to turn on/off for an entire timestep in one go