add new model ordinal_reg() for ordinal regression

[corybrunson]

This was discussed in two previous issues, #35 and #953. The new model is being added in this fork branch, and an extension with engines is underway here.

First see this article for guidance on adding a new model.

Checklist for adding a new model

Copied from this article:

• Create the main modeling function (e.g. nearest_neighbors()). When choosing argument names, check for existing ones across other model types which could be reused, e.g., trees for the number of trees is used across different tree-based models like rand_forest() and boost_tree().

  • here
  • Model parameters currently include penalty, mixture, ordinal_link, and odds_link.

• Register the model and a mode in the parsnip model “database” with parsnip::set_new_model() and parsnip::set_model_mode(). [example]

  • specification only

• Create an update() method.

  • here

• Add parameter objects for main parameters to dials, if needed. See How to create a tuning parameter function for more details.

  • here
  • This is done in a coordinated fork branch.

• Write unit tests for the basic modeling function (independent of the engines).

• Add a pkgdown entry.

• Add model description to the internal model_descs tibble.

• Run purrr::map(parsnip:::extensions(), ~ library(.x, character.only = TRUE)).

• Run parsnip:::update_model_info_file().

• Make sure that no entries are removed from inst/models.tsv.

• Add a documentation entry in NEWS.md.

[corybrunson]

One question i have is when ordinal_reg() engines should be added to model_info_table—presumably once {ordered} is on CRAN? This seems to be a prerequisite for the informative error message produced by prompt_missing_implementation().

[corybrunson]

The last several items (except NEWS.md) produce no changes when {ordered} is not installed included in parsnip:::extensions(), so i have the same question as above: Should the table be updated with the {ordered} engines before or after {ordered} is published on CRAN?

[corybrunson]

Should writing the .Rmd engine documentation files, as described in 'inst/README-DOCS.md', be added to this checklist? These are minimally done but need to be updated with model specifics.