Refactor local_ensemble mask calculations

[eivindjahren]

While working on #7046 , some refactoring was found to be beneficial. Separating this out here.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5344f25) 84.71% compared to head (fcadc88) 85.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7197      +/-   ##
==========================================
+ Coverage   84.71%   85.43%   +0.72%     
==========================================
  Files         380      380              
  Lines       22633    22626       -7     
  Branches      931      932       +1     
==========================================
+ Hits        19174    19331     +157     
+ Misses       3340     3178     -162     
+ Partials      119      117       -2     

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