Issue #559: Ensure output of add_coverage() is an object of class forecast_quantile

[nikosbosse]

Description

This PR closes #559.

add_coverage() used to return a data.table(), even if it was called on an object of class forecast_quantile. This was due to internal computations: the function quantile_to_interval() returns a data.table and when merging the newly computed columns with the existing data, a data.table was produced.

In addition, transform_forecasts() had a similar issue. This was due to a call to as.data.table().

This PR

• switches the order in which the two datasets are passed to merge(), solving the issue for add_coverage()
• replaces as.data.table() with as_forecast() for transform_forecasts()
• Adds a unit test for both
• Adds an additional, mostly unrelated unit test to check that running as_forecast() twice produces the same object. I added this test because add_coverage() currently runs as_forecast() and I thought it was a good idea to have a test that running as_forecast() several times is fine.
• removes unrelated data.table:: to make the code style more consistent

Additional thoughts:

• we should likely check and test all calls to as.data.table() in the code. All of these would make an object lose its classes. I opened Check all calls to `as.data.table() to make sure they are not dropping classes we want to keep #587 for this.

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 (61905cf) 85.15% compared to head (14de707) 85.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
- Coverage   85.15%   85.14%   -0.01%     
==========================================
  Files          21       21              
  Lines        1711     1710       -1     
==========================================
- Hits         1457     1456       -1     
  Misses        254      254              

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[seabbs]

LGTM.