Meta Issue: Clean-up of current major changes

[nikosbosse]

Need to clean up bits and pieces:

• Rework arguments for find_duplicates #295
• Add @return to the documentation of every function #327
• Sunset deprecated argument in summarise_scores() #300
• Make sure functions are consistently called check_, test_, assert_, get_. #357
• Update Vignettes
• make sure all files correspond to linter requirements
• make sure the internal keywords make sense for pkgdown
• make sure that all error messages on input checks are really informative.
• Clean up deprecate version number, e.g. there is line "1.2.2", "avail_forecasts()",
• Check that functions have match.arg checks for their arguments
• Clean up random spaces in code and make that consistent
• Review tests
  • make sure tests are in the correct file (for point metrics and binary metrics, files could be organised more cleanly)
  • make sure tests make sense
  • check whether we have duplicate computations of the same thing
• check behaviour of add_coverage() and add_pairwise_comparison(). The latter sets by to "model" if it is equal to the forecast unit, the former doesn't do it at the moment.

In terms of error messages, something that we had before that I liked:

sprintf(
        "Mismatch: 'observed' has length `%s`, but 'predicted' has length `%s`.", # nolint
        length(observed), length(predicted)
      )

or

 if (!is.null(observed) && nrow(predicted) != length(observed)) {
      msg <- sprintf(
        "Mismatch: 'observed' has length `%s`, but 'predicted' has `%s` rows.",
        length(observed), nrow(predicted)
      )
      stop(msg)
    }

Making sure that arguments are called the same everywhere