Issue #494: New workflow for creating and validating forecast objects

[nikosbosse]

Description

This PR closes #494.

This PR

• replaces the function validate.default() with a new class construction helper as_forecast(), a function that determines the forecast type, constructs a new class object, and validates that object. as_forecast() is a generic with a single method as_forecast.default() that does the work
• Replaces all instances of validate() (in the sense of the previous validate.default()) in the code, the news file, and the tests
• Reorganises the documentation for score() and as_forecast() a bit, such that a section that is common to both can be reused
• adds the .trackdown folder to .gitignore and .buildignore - the changes suggested in Implement updated validation and class construction workflow #494, but I recently used trackdown on a different branch (but also gitignored it so I can't just delete the file) for updates to the manuscript
• Renamed the generic validate() to validate_forecast(). The class construction helper (which was previously validate.default() is therefore now as_forecast() and the validation function (previously the methods of validate()) are now methods of validate_forecast()

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c11ce14) 82.44% compared to head (f7ece21) 82.47%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #531      +/-   ##
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+ Coverage    82.44%   82.47%   +0.03%     
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  Files           20       20              
  Lines         1675     1678       +3     
===========================================
+ Hits          1381     1384       +3     
  Misses         294      294              

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[seabbs]

This all looks good to me. Confused about the use of scoringutils_binary etc. and I think could do with a bit more checking the as_forecast workflow makes sense and is used clearly.

[seabbs]

Can you resolve and request another review when ready please!

[nikosbosse]

I updated score.default() to not validate forecasts. I also updated the Vignette to make use set_forecast_unit() and as_forecast() consistently.

We'll likely need another round of edits for the Vignette. I suggest to not mix that with this PR. I haven't finished updating the scoringutils manuscript yet and I think we can re-use parts of that, creating a coherent structure that makes sense. I opened #532 to keep track of this.

Side note: I also made a new comment to #435 since I noticed that the current default for by in summarise_scores() maybe doesn't that make much sense anymore now that we're not returning scores per quantile anymore.

PR is good for another round of review. Thanks a lot!

[seabbs]

Nice. All makes sense. Looks good to me.