Improve GHC for points beyond nmax

[juliohm]

Instead of using a NN model on geospatial coordinates, use a NN model on the features.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.12500% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.55%. Comparing base (3cd21b8) to head (702ddec).

Files with missing lines	Patch %	Lines
src/clustering/ghc.jl	53.12%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   94.95%   93.55%   -1.40%     
==========================================
  Files          21       21              
  Lines         932      947      +15     
==========================================
+ Hits          885      886       +1     
- Misses         47       61      +14     

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