Fix inplace renaming of columns

[nikosbosse]

Description

This PR closes #979.
This PR closes #972 .

This PR fixes the issues above by handling all renaming of columns in as_forecast_generic(). The code there became a bit more complicated, but on the other hand the individual as_forecast_() functions got simplified to the point that they become almost trivial. In a later PR we could really internalise all the logic into as_forecast_generic() and only pass in the class name as an arg.

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]

Nice LGTM. Just needs a news entry

[nikosbosse]

Ha I hoped you would let me get away since it's just an internal thing :D I'll add a news item