Issue #447 - Add input checks to functions

[nikosbosse]

Description

As mentioned in #447, a few functions currently lack input checks. This PR adds input checks to various functions. Notable changes:

• updated the metrics default argument for get_correlations() to get_metrics(scores)
• get_forecast_unit() now errors if the input is not a data.frame. This means I also had to update print() to add a try()-statement around get_forecast_unit() and update the messages slightly

To avoid another mega-PR, I looked at all functions up to and including metrics-validate.R. I'll do the rest in a second PR.

Reminder for future me: look at docs for quantile_score() once again and see whether arguments are documented correctly.

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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.30%. Comparing base (a8ae1d0) to head (c2e0d49).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #748 +/- ## ========================================== + Coverage 95.27% 95.30% +0.02% ========================================== Files 21 21 Lines 1588 1596 +8 ========================================== + Hits 1513 1521 +8 Misses 75 75 ```

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