DM-55875: Implement Gomes 2025 with the help of Claude/Fable5.

[PFLeget]

With the help of Claude (Fable5), this PR implementing the method described in Gomes et al. 2025. The idea is to have an empirical kernel that is derived directly from the measured 2D two-point correlation function and so get free of model choice for the kernel and hyper parameters fitting.

[codecov]

Codecov Report

❌ Patch coverage is 95.74037% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.74%. Comparing base (412e527) to head (dae683e).

Files with missing lines	Patch %	Lines
treegp/empirical_2pcf.py	93.81%	12 Missing ⚠️
treegp/kernels.py	94.20%	4 Missing ⚠️
treegp/gp_interp.py	96.29%	3 Missing ⚠️
treegp/grid_gp.py	98.63%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   89.13%   91.74%   +2.60%     
==========================================
  Files           9       11       +2     
  Lines         939     1429     +490     
==========================================
+ Hits          837     1311     +474     
- Misses        102      118      +16     

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[PFLeget]

Going to merge this to try to make rubin-env 14.