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: Make sample_to_quantile() require a forecast object of class forecast_sample #755

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.29%. Comparing base (8534795) to head (f70bf30).

Additional details and impacted files

@@            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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[seabbs]

Nice