Add OpenMP to build and enable 5-D multilinear interpolation

[djhoese]

Claude pointed out that 5D interpolation wasn't enabled in the multilinear module, something I'd noticed before. Claude also pointed out that multilinear uses prange which is an OpenMP multithreading pragma, but geotiepoints was never built with the necessary openmp flags to use it. Claude's understanding is that this isn't a problem and the interpolation should work like this. If the original author of these interpolators knows (@mraspaud @adybbroe ?) for sure, it'd be great to nail this down.

Claude's benchmark results:

Timing (d=4, orders=[20]⁴, 2 value rows, 2M points): 81.8 ms at 1 thread → 21.4 ms at 8 (~3.8×).

I told Claude to pick OpenMP knowledge from pykdtree where OpenMP has been a feature for a long time.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.26%. Comparing base (e74a0ef) to head (b2b878b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
geotiepoints/multilinear_cython.pyx	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   89.04%   94.26%   +5.22%     
==========================================
  Files          20       20              
  Lines        1515     1605      +90     
==========================================
+ Hits         1349     1513     +164     
+ Misses        166       92      -74     

Flag	Coverage Δ
unittests	94.26% <96.42%> (+5.22%)	⬆️

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