Add power_spectral_density to RatQuad covariance kernel for HSGP support

[fonnesbeck]

Description

PSD method and test added. A simple HSGP fits successfully with the resulting RatQuad kernel.

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.49%. Comparing base (f69e159) to head (4840b88).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7973   +/-   ##
=======================================
  Coverage   91.49%   91.49%           
=======================================
  Files         116      116           
  Lines       18962    18971    +9     
=======================================
+ Hits        17349    17358    +9     
  Misses       1613     1613           

Files with missing lines	Coverage Δ
pymc/gp/cov.py	98.68% <100.00%> (+0.02%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.