Issue #761 - Update input checks and docs for `quantile_score()`

[nikosbosse]

Description

This PR closes #761.

Previously, the function quantile_score() inherited its arguments from wis(), but the actual format it expected was different from the one used by the other quantile-based metrics. The function used to expect a vector of observed values and a single vector with predicted values and a single quantile level. This is in contrast to the other quantile-based functions, which usually expect a matrix of predicted values and are also able to accept several quantile levels.

This PR changes the expected input format of quantile_score() such that it conforms to what the other functions accept. Instead of computing the quantile score for a single vector, it now computes the score for several quantile forecasts and returns an average score. One implication of this is that the output of wis() and quantile_score() are now mostly identical, apart from the fact that

• wis() can only compute something if two quantiles form a prediction interval
• quantile_score() can compute a score for a single quantile level, but doesn't offer the decomposition

The PR

• adds new input checks to quantile_score()
• rewrites the code to work with matrices instead of vectors
• taught me about the existence of the sweep() function (admittedly I'm still not entirely sure what it does). R magic of the ancients, forged before the time of man.
• adds a new unit test to test the new behaviour
• updates the documentation

I haven't added a news item because the update is changing the input formats for all functions (we just hadn't properly implemented that change for quantile_score() yet)

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 95.29%. Comparing base (84cbcc9) to head (96c1010).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #768 +/- ## ======================================= Coverage 95.29% 95.29% ======================================= Files 21 21 Lines 1594 1595 +1 ======================================= + Hits 1519 1520 +1 Misses 75 75 ```

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