Issue ##438 - Add tests to check existing data.table functions work with data.frames

[nikosbosse]

Description

This PR closes #438.

This PR adds additional tests to make sure that all exported functions that accept a data.table also work with a data.frame. I simply went through a list of the functions and added tests.

I got the list of functions to check as such (thanks, GPT):

exported_functions <- ls("package:scoringutils")
functions_with_data_arg <- character(0) 
for (func_name in exported_functions) {
  func_args <- suppressWarnings(formals(get(func_name)))
  if (!is.null(names(func_args)) && names(func_args)[1] == "data") {
    functions_with_data_arg <- c(functions_with_data_arg, func_name)
  }
}
functions_with_data_arg

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [ ] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.