Convert ARX forecaster to the correct format

[dajmcdon]

The proper forecaster signature should be:

prob_arx <- function(x, y, geo_value, time_value, args = prob_arx_args())

All args not in the epi_ data frame should be in an args list.

[brookslogan]

I think we may want to consider instead

function(df, args = prob_arx_args())

or something mirroring fable, whatever its interface is.

As:

• More commonly than not, users will probably will want (x,y,geo_value,time_value) in a data frame format to pass to a fitter function like lm, rq, etc. If they are given df as an input, this is less trouble.
• The code to call a forecaster / list of forecasters seems easier:

fetch_latest_data_from_my_favorite_api() %>% prob_arx(my_args)
as_of(archive, forecast_date) %>% prob_arx(my_args)
snapshot = as_of(archive, forecast_date)
lapply(forecasters_with_args, function(forecaster_with_args) {
  forecaster_with_args$forecaster(snapshot, forecaster_with_args$args)
})

as opposed to something with do.call or tidy_eval or things along those lines. Although I'm not sure if taking df as an argument would make things easier or harder when using epi[x]_slide; I forget if slider expects functions to take in the columns or the df.