Reduce the number of attributes used by validate_forecast()

[nikosbosse]

Description

This PR fixes #515, fixes #456.

The PR

• removes the attributes warnings, forecast_unit, and forecast_type from the output of validate_general()
• replaces all instances where functions relied on a forecast_unit attribute by calls to get_forecast_unit()
• removes a check_conflict option from get_forecast_unit(), which was there to check for a conflict between stored attribute and inferred forecast unit
• removes a superfluous 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 ✅

Comparison is base (c811d55) 83.74% compared to head (d7c64b3) 83.69%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #541      +/-   ##
===========================================
- Coverage    83.74%   83.69%   -0.06%     
===========================================
  Files           21       21              
  Lines         1723     1717       -6     
===========================================
- Hits          1443     1437       -6     
  Misses         280      280              

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[seabbs]

Nice. LGTM