Issue #520: Rename forecast classes from scoringutils_* to forecast_*

[nikosbosse]

Description

This PR is related to #520.

Mostly does what it says on the tin:

• Replaces all instances of scoringutils_binary etc. with forecast_binary etc.
• updates the name of the constructor function from new_scoringutils() to new_forecas() and updates that function as well
• unrelated to the mentioned issue, there was a roxygen warning about the topic validate (the function had been renamed to validate_forecast() in Issue #494: New workflow for creating and validating forecast objects #531). This PR fixes this by updating the @rdname tag.
• updates the News.md 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a7d12d1) 81.72% compared to head (9309441) 82.50%.
Report is 35 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #533      +/-   ##
===========================================
+ Coverage    81.72%   82.50%   +0.77%     
===========================================
  Files           20       20              
  Lines         1702     1680      -22     
===========================================
- Hits          1391     1386       -5     
+ Misses         311      294      -17     

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[seabbs]

LGTM. Getting there!