Use exponentially scaled modified Bessel function in periodic HSGPs

[dehorsley]

Description

This updates the scale factor in the periodic HSGP implementation to use the exponentially scaled modified Bessel function recently added to PyTensor. This should improve the numerical stability of the implementation, and simplify the gradient slightly. Note

ive(v, z) = iv(v, z) * exp(-abs(z.real))

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7228.org.readthedocs.build/en/7228/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.30%. Comparing base (120fc4e) to head (3060594).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7228   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files         100      100           
  Lines       16895    16895           
=======================================
  Hits        15595    15595           
  Misses       1300     1300           

Files	Coverage Δ
pymc/gp/cov.py	98.66% <100.00%> (ø)

[bwengals]

thank you @dehorsley good find