Added update.layer and update_frosting

[rachlobay]

Addresses #73. Added update.layer() and update_frosting() along with tests for each.

[rachlobay]

Actually, hold up... I see that workflows update_recipe() and remove_recipe() convert the epi_workflow to just a workflow unfortunately... And I expect that their update_model() function will have a similar result... I will have to add the equivalents to work with an epi_workflow(). It's probably best to try get this done all on one PR (as they are very similar tasks), so I will let you know/post here after I've done this.

[rachlobay]

Update: I’ve finished adding functions (+ tests) for add/update/remove a model and a recipe that work with an epi_workflow. Note that I did not add functions to update a recipe step or model specification in a parsnip model because the existing functions for those seem to work ok.

What I mean by the last sentence can be seen through the following example:

library(epiprocess)
library(dplyr)
library(recipes)

jhu <- case_death_rate_subset %>%
  filter(time_value > "2021-08-01") %>%
  dplyr::arrange(geo_value, time_value)

r <- epi_recipe(jhu) %>%
  step_epi_lag(death_rate, lag = c(0, 7, 14)) %>%
  step_epi_ahead(death_rate, ahead = 7) %>%
  step_epi_lag(case_rate, lag = c(0, 7, 14)) %>%
  step_naomit(all_predictors()) %>%
  step_naomit(all_outcomes(), skip = TRUE)

# Update a recipe step using the tidymodels function
# https://github.com/tidymodels/recipes/blob/HEAD/R/update.R
r2 <- r
r2$steps[[1]] <- update(r2$steps[[1]], lag = 4)

# Update model specification for a parsnip model using the parsnip function
# https://parsnip.tidymodels.org/reference/parsnip_update.html
mod1 <- rand_forest(mode = "regression", trees = 10)

update(mod1, trees = 20)

[dajmcdon]

1. One structural issue which we can discuss.
2. A few minor style issues.
3. There's a stray file R/add_model.Rd to remove.

[dajmcdon]

I added a number of comments. The ideas are good but a few major changes:

• Add a tidy() method for frosting objects.
• Allow the adjust methods to handle names as well as numbers.
• Create a short vignette that illustrates how these can be used. (including the tidy() methods for examining the recipe/frosting).
• Run styler so that the code formatting matches the style on main.