Issue #750 - Introduce a forecast_type argument in validate_forecast()

[nikosbosse]

Description

As discussed in #750, it would be nice to have a way in validate_forecast() to check the type is exactly what you want.

This PR therefore

• introduces a forecast_type argument in validate_forecast(), analogously to the one that already exists in as_forecast().
• creates a new function, assert_forecast_type() that does exactly that, simplifying the code in as_forecast() and minimising repetitive code in validate_forecast(). The rationale for the slightly odd design with actual and desired is that that avoids running get_forecast_type() again in situations where the forecast type is already determined by the class of the object. An earlier version only had a single argument forecast_type() for the desired type and the actual type was determined automatically. This, however, presumes that validation works. I think both versions are fine if you have a preference 🤷
• creates a new test

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 ✅

Project coverage is 95.37%. Comparing base (1a51fa8) to head (232a832).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #751      +/-   ##
==========================================
+ Coverage   95.36%   95.37%   +0.01%     
==========================================
  Files          21       21              
  Lines        1617     1623       +6     
==========================================
+ Hits         1542     1548       +6     
  Misses         75       75              

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[seabbs]

These cli messages are so pretty.

This, however, presumes that validation works. I think both versions are fine if you have a preference 🤷

Yeah I really have no preference