feat: add GP-age epigenetic clock

[marcbal77]

Summary

• Add GP-age clock (Gaussian Process regression-based age prediction)
• 6 model variants: GPAge10, GPAge30, GPAge71, GPAgeA, GPAgeB, GPAgeC
• GPy added as optional dependency (pip install biolearn[gpage])

Reference

• Paper: https://www.cell.com/cell-reports-methods/fulltext/S2667-2375(23)00211-4
• GitHub: https://github.com/mirivar/GP-age
• License: MIT (confirmed with authors)

[marcbal77]

Note: The .json.zip extension is misleading - they're actually gzip-compressed JSON files. GPy handles this natively. I noticed this is different than the linear clocks managed as csv's.