Issue #832 - Make default metrics function S3

[nikosbosse]

Description

This PR closes #832.

Creates a single get_metrics() generic with S3 methods for forecast and scores objects.

Additional comments:

• still need to update the news file again
• could create a default method that just says "error, needs a forecast or scores object". But then again R already warns you that there is no appropriate method
• I'm planning to move all functions to respective files for the different forecast types. This should make adding more forecast types in the future easier. But I thought it would be easier for review to leave as is for now and tackle it in a different PR.

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.

[seabbs]

But I thought it would be easier for review to leave as is for now and tackle it in a different PR.

Good call

[seabbs]

I really like how this has turned out. I can think of a few uses cases where this might annoy people (if they don't have an object of the right class but want the list) but think that overall should be a much better user experience.

[nikosbosse]

I can think of a few uses cases where this might annoy people (if they don't have an object of the right class but want the list)

yeah me too. In that sense I think changing the examples to pre-validated was a good move