refactor model specification printing

[simonpcouch]

While working on some previous PRs, I've noticed that all but one or two of our model spec print methods take the form:

print.bag_mars <- function(x, ...) {
  cat("Bagged MARS Model Specification (", x$mode, ")\n\n", sep = "")
  model_printer(x, ...)

  if (is_printable_spec(x)) {
    cat("Model fit template:\n")
    print(show_call(x))
  }
  invisible(x)
}

where the only line that changes method-to-method is the cat header summarizing the model type.

This PR refactors model specification print methods to all dispatch to print.model_spec, which now makes use of a print_model_spec helper that subsets a tibble giving the descriptions for each class to change that model type description in the header. Hopefully we find this to be more maintainable.

This change should be non-breaking for dependencies.

• If they define engines for a model type that is already supported in parsnip and rely on parsnip's print methods, we've tested that the output doesn't change. (Exceptions noted in 135a544—"Model " is added preceding "Specification" in the header in 4 print methods.)
• If they supply their own print methods, print.model_spec won't be dispatched to.

This PR still exports the new print_model_spec helper, though, in case dependencies want to make use of it. That code would look like:

print_model_spec(new_type_object, desc = "New Type Description")

---

This PR will be hard to review as a whole (lots of deletions across many files), so I tried to commit in such a way to make reviewing commit-by-commit much easier. :)

[EmilHvitfeldt]

This is related to #567

[simonpcouch]

Oh, sweet! Closes #567. You noted that same edge case, too. :)

[hfrick]

Nice one! Thanks for the commit order, that made it a lot easier indeed 🙌

Emil's comment #567 (comment) about the print method for the null model is still open. If we want this PR to close #567, we should address this or turn it into its own issue so that it doesn't get lost.

[simonpcouch]

Ah, thanks for pointing that out.

It looks like the output of null_model doesn't actually dispatch to the print method mentioned in #567. All of the methods for null models are defined for class nullmodel, but the output of null_model has first class null_model. With the current PR (that doesn't adjust null model internals):

library(parsnip)

null_mod <- null_model()

class(null_mod)
#> [1] "null_model" "model_spec"

methods(class = "null_model")
#> no methods found
methods(class = "nullmodel")
#> [1] predict print   tidy   
#> see '?methods' for accessing help and source code

# dispatches to new method :(
null_mod
#> null model Model Specification (classification)

^{Created on 2022-06-01 by the reprex package (v2.0.1)}

I could print.null_model <- print.nullmodel here, but I think a fresh issue re: null model dispatch is probably more appropriate.

[hfrick]

I think nullmodel is the class for a fitted null model, while null_model is the class for the model spec. Does adding a row to model_descs for the null model solve the unfortunate printing for null_model()?

[simonpcouch]

It does🐨

library(parsnip)

null_model()
#> Null Model Specification (classification)

^{Created on 2022-06-01 by the reprex package (v2.0.1)}

[simonpcouch]

Thanks for the review, @hfrick! :) Ready to merge from our perspective, @topepo.

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