Use class() and attr() instead of structure()

[jamesmbaazam]

This PR fixes #183 by replacing the use of structure() to construct a class. The lintr suggests using a call to class() followed by calls to attr().

This PR will be merged ahead of #182 to fix the CI warnings there.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b5a3007) 99.26% compared to head (daa2e19) 99.26%.
Report is 1 commits behind head on main.

❗ Current head daa2e19 differs from pull request most recent head 62e522c. Consider uploading reports for the commit 62e522c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   99.26%   99.26%   -0.01%     
==========================================
  Files           8        8              
  Lines         684      682       -2     
==========================================
- Hits          679      677       -2     
  Misses          5        5              

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