Fix multi-threaded interpolation

[juliohm]

Fix JuliaEarth/GeoStats.jl#543

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.58%. Comparing base (1b84167) to head (ace7aee).

Files with missing lines	Patch %	Lines
src/models.jl	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   95.57%   95.58%   +0.01%     
==========================================
  Files          11       11              
  Lines         542      544       +2     
==========================================
+ Hits          518      520       +2     
  Misses         24       24              

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