Create forceast super class

[nikosbosse]

Description

This PR closes #.

As discussed in #791, this PR adds an additional forecast super class and replaces the current print method with it. I also slightly updated an error message.

Missing:

• news items
• docs
• paper

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
• [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.