Issue #672 - Fix `get_correlations()`

[nikosbosse]

Description

This PR closes #672.

I noticed a weird error in #672. The error comes from the fact that if you passed unsummarised scores to get_correlations() then it removed the coverage columns because those are not numeric.

This PR

• drops the code that does that and lets cor() handle any exceptions.
• adds unit tests with something that would previously have produced an error
• adds a ... argument to get_correlations(). Not sure how many people will actually want to pass additional args to cor() but then again why not...

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.
• [x] 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 95.26%. Comparing base (f7b5283) to head (a9d069b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #726 +/- ## ========================================== - Coverage 95.27% 95.26% -0.01% ========================================== Files 21 21 Lines 1630 1627 -3 ========================================== - Hits 1553 1550 -3 Misses 77 77 ```

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