Issue #851 - Create S3 methods for converting to point and quantile forecasts

[nikosbosse]

Description

This PR closes #851.

The PR

• creates S3 methods for as_forecast_point() and as_forecast_quantile to convert quantile-based and sample-based forecasts
• splits up the doc files for as_forecast(). ?as_forecast() now has the overview of all the types and there are separate entries for all the specific as_forecast_<type>() functions. In the future we could flesh them out more, but at the moment it's not top on my priority list
• updates tests and the news file.

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]

This is great. The new interface is starting to feel really slick. A few comments about the s3 setup that can be handled in another issue and a few depreciated arg that have slipped through

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.20%. Comparing base (4fee240) to head (1904f52).
Report is 43 commits behind head on main.

Files	Patch %	Lines
R/forecast.R	76.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
- Coverage   95.47%   95.20%   -0.27%     
==========================================
  Files          21       21              
  Lines        1568     1606      +38     
==========================================
+ Hits         1497     1529      +32     
- Misses         71       77       +6     

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[nikosbosse]

I think I successfully addressed all points by deferring them to new issues :D
Good to merge?

[seabbs]

Yup