Issue #443 Drop interval functions

[nikosbosse]

Description

This PR closes #443.

The previous version 1.2.2 of scoringutils exported function for an interval format that is no longer supported. This PR

• therefore drops the function bias_range().
• It also makes interval_score() an internal function (it is used within wis()).
• deletes unnecessary tests and creates a new test for bias_quantile() that tests whether the function can be used for a point forecast.
• Updates examples for interval_score() in the Vignettes and a copy of that example was added to wis() which didn't have any examples yet.
• solves a small linting issue (removing an unnecessary call to ggplot2 in the Vignette)

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 ✅

Comparison is base (97d204f) 81.21% compared to head (ab36bae) 81.82%.
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #525      +/-   ##
===========================================
+ Coverage    81.21%   81.82%   +0.61%     
===========================================
  Files           20       20              
  Lines         1725     1695      -30     
===========================================
- Hits          1401     1387      -14     
+ Misses         324      308      -16     

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[seabbs]

LGTM