Updated cuminc() and crr() to drop un-observed factor levels

[ddsjoberg]

What changes are proposed in this pull request?

• Updated cuminc() and crr() to drop un-observed factor levels before processing the data. Previously, un-observed levels led to a singularity error.

If there is an GitHub issue associated with this pull request, please provide link.
closes #76
closes #59

---

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 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
• usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

• Update NEWS.md with the changes from this pull request under the heading "# tidycmprsk (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end of the update.
• Increment the version number using usethis::use_version(which = "dev")
• Run usethis::use_spell_check() again
• Approve Pull Request
• Merge the PR. Please use "Squash and merge".

[tengfei-emory]

The singularity issue is addressed for example data.