Issue #275: Fix handling of scalar inputs in logs_binary()

[nikosbosse]

Description

This PR closes #275.

Ideally we would like logs_binary() to be able to accept a single value as an input that then gets recycled just like any other vector in R does. The current implementation had an ifelse() statement there which did work for several observed values and a single predicted value, but not the other way round.

This PR updates the code in logs_binary() to work with scalar values and adds tests against the function ll() from the Metrics package to verify everything is correct.

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.
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• 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 (97d204f) 81.21% compared to head (2365a22) 81.21%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #524   +/-   ##
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  Coverage    81.21%   81.21%           
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  Files           20       20           
  Lines         1725     1725           
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  Hits          1401     1401           
  Misses         324      324           

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[seabbs]

LGTM