Add setter for FoamFieldFile.boundary_field property

[gerlero]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.56%. Comparing base (b0b1a87) to head (65a1878).

Files with missing lines	Patch %	Lines
foamlib/_files/_files.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
- Coverage   88.61%   88.56%   -0.06%     
==========================================
  Files          16       16              
  Lines        1239     1242       +3     
==========================================
+ Hits         1098     1100       +2     
- Misses        141      142       +1     

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