Fix call to model_cfg to use getter function

[lewisblake]

Change Summary

ModelMapsEngine had not been updated to use the getter function for model_names.

Related issue number

NA, see chat with Charlie

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

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.58%. Comparing base (aa4c23e) to head (8d55c2b).
Report is 3 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/aeroval/modelmaps_engine.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1424      +/-   ##
============================================
+ Coverage     78.35%   78.58%   +0.22%     
============================================
  Files           137      137              
  Lines         20889    20889              
============================================
+ Hits          16367    16415      +48     
+ Misses         4522     4474      -48     

Flag	Coverage Δ
unittests	78.58% <80.00%> (+0.22%)	⬆️

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