Mice nnet

[ddsjoberg]

What changes are proposed in this pull request?
mice and nnet mutlinomial models can now be used in tbl_regression().

• PR implements a tidy method for mice (mira) objects. the tidy method performs the mice::pool() portion AND the mice::tidy() part. The reason for this is that the pooling function removes the model objects, and returns a tibble of the summarized model. We can access one of the underlying models build on one of the imputed sets so we can extract the variable names, contrasts, etc.
• PR includes a tbl_regression method for nnet multinomial models. The underlying dataset is one line per level of the outcome per term in the model. This is not the typical structure. I added a note indicating the other tbl_regression functions may yield unexpected results.

If there is an GitHub issue associated with this pull request, please provide link.
closes #645

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Checklist for PR reviewer

• PR branch has pulled the most recent updates from master branch. Ensure the pull request branch and your local version match and both have the latest updates from the master branch.
• If an update was made to tbl_summary(), was the same change implemented for tbl_svysummary()?
• If a new function was added, function included in _pkgdown.yml
• If a bug was fixed, a unit test was added for the bug check
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• Code coverage is suitable for any new functions/features. Review coverage with covr::report(). Before you run, set Sys.setenv(NOT_CRAN=true) and begin in a fresh R session without any packages loaded.
• R CMD Check runs without errors, warnings, and notes
• NEWS.md has been updated with the changes from this pull request under the heading "# gtsummary (development version)". If there is an issue associated with the pull request, reference it in parantheses at the end update (see NEWS.md for examples).
• usethis::use_spell_check() runs with no spelling errors in documentation
• When the branch is ready to be merged into master, increment the version number using usethis::use_version(which = "dev"), run codemetar::write_codemeta(), approve, and merge the PR.

[ddsjoberg]

@karissawhiting @michaelcurry1123 this one is also available, if you'd like 🍁

[michaelcurry1123]

@ddsjoberg I am up for it

[ddsjoberg]

@ddsjoberg I am up for it

Thanks @michaelcurry1123 ! Would you be able to start with #685 ?

[michaelcurry1123]

@ddsjoberg never been more up for it!

[ddsjoberg]

@michaelcurry1123 haha yay!