Issue #355 - Add input checks to coverage plots

[nikosbosse]

Description

Adds simple input checks to the coverage plot functions (making sure input is a data.frame and colour is a column of the data).

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 95.36%. Comparing base (a5982e9) to head (98c251f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #734 +/- ## ======================================= Coverage 95.35% 95.36% ======================================= Files 21 21 Lines 1658 1661 +3 ======================================= + Hits 1581 1584 +3 Misses 77 77 ```

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