Fix set equivalence checking in ModelMapsEngine

[lewisblake]

Change Summary

Title. @jgriesfeller noticed this bug.

Related issue number

NA

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[jgriesfeller]

all good

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.83%. Comparing base (d40203e) to head (ef31aeb).
Report is 10 commits behind head on main-dev.

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1357      +/-   ##
============================================
+ Coverage     78.68%   78.83%   +0.15%     
============================================
  Files           136      136              
  Lines         20787    20788       +1     
============================================
+ Hits          16357    16389      +32     
+ Misses         4430     4399      -31     

Flag	Coverage Δ
unittests	78.83% <ø> (+0.15%)	⬆️

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