Redefine example forecasters to use control lists

[ryantibs]

In the slide vignette, archive vignette, and advanced slide vignette, we should rewrite the example forecaster prob_ar() or prob_arx() so that all of its arguments outside of data (the variables used to produce the forecast) are passed through a control list.

That is how we're currently thinking about setting things up in epiprocess so it'll make sense to set things up here for unification purposes.