Warn if 'mean' padding mode is used for label maps

[fepegar]

Resolves #1015.

Description

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.30%. Comparing base (c094a18) to head (d76590b).
Report is 83 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1065      +/-   ##
==========================================
+ Coverage   87.29%   87.30%   +0.01%     
==========================================
  Files          91       91              
  Lines        5800     5806       +6     
==========================================
+ Hits         5063     5069       +6     
  Misses        737      737              

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