#931 - Fix data.table issue caused by attr()

[nikosbosse]

Description

This PR closes #931

As discussed in #931, manipulating scores objects caused a warning due to the use of attr() in [.scores. This PR replaces attr() with setatrr() and adds some tests.

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.
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[seabbs]

Nice catch!