Issue #447 - Add even more input checks

[nikosbosse]

Description

As discussed in #447, some functions lack input checks. This is the third PR that adds more checks.

This PR

• simplifies input checks in summarise_scores()
• slightly updates the error message in get_metrics() + associated tests
• adds input checks to
  • merge_pred_and_obs()
  • sample_to_quantile()
  • run_safely()
• replaces some data.table:: calls with ensure_data.table()

Further thoughts:

• there is an argument to be made that sample_to_quantile() should require a validated forecast_sample. I opened a separate issue for this: https://github.com/epiforecasts/scoringutils/issues/755

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.
• [x] 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 95.29%. Comparing base (8534795) to head (f70bf30).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #756 +/- ## ========================================== - Coverage 95.30% 95.29% -0.01% ========================================== Files 21 21 Lines 1596 1594 -2 ========================================== - Hits 1521 1519 -2 Misses 75 75 ```

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