Updates for new aorsf engine for rand_forest()

[bcjaeger]

Closes #827

Hello!

I would like to include the oblique random survival forest in parsnip as an engine for censored regression. I have reviewed the guidelines for contributing engine level documentation and have made .Rmd and .R files for aorsf that mimic existing files for random forest engines, e.g., partykit.

Following the engine-specific guidelines in inst/README-DOCS.md,

1. Use roxygen comments to generate the main .Rd file that R's help system uses.
2. Create an .Rmd file that has the details of the engine.
3. Knit the .Rmd file to .md so the .Rd file from step 1 can link it.
4. Once you are done, run devtools::document() (or use the RStudio IDE) to build the help files.

I had a little trouble on step 3. Knitting my .Rmd file, the error package 'parsnip' required by 'censored' could not be found occurred. So I did not commit the .Rd file. I've tried to figure out what is causing parsnip to be undetected, but haven't figured it out yet.

If you make a PR with a new engine, you can work on the .Rmd file and the tidymodels folks can go through the process of merging it in (but let us know if you want us to do that in the PR).

Since aorsf is a new engine and I am having trouble creating the .Rd file, may I request help getting the documentation files to knit as intended?

[hfrick]

@topepo can you have a look at this, too? This PR now contains all the changes related to the new aorsf engine for censored regression with rand_forest():

• update to the tunable() method for an engine-specific argument
• a fit_xy() method for random forests: this is so that it can error properly for this engine. I've opened an issue for it to discuss with @bcjaeger (Hi! 👋) if aorsf could change to enable that Enable single Surv object as the reponse in a formula for orsf() ropensci/aorsf#11
• the engine docs

The corresponding PR on censored is tidymodels/censored#211 if you want more context.

[hfrick]

• todo for myself: add a note about case weights to the docs

[hfrick]

@topepo this is ready to merge 👍

fit_xy() now works with the dev version of aorsf thanks to @bcjaeger changes in aorsf but I'm leaving fit_xy.rand_forest() in parsnip until the dev version of aorsf makes it to CRAN.

[bcjaeger]

Thank you! 😃

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.