gh-563: Add Pcb support for Cl computations in massive neutrino cosmologies

[matteobaratto]

🚀 Pull Request Checklist

✅ Summary

The Pcb matter power spectrum has already been implemented in the library. However, the Cl computation pipeline currently always uses the total matter power spectrum (Pmm) and does not expose the possibility of selecting an alternative power spectrum.

Some applications, such as cosmological analyses including massive neutrinos, require different choices of matter power spectrum depending on the observable. For example, galaxy clustering may require the use of the cold dark matter + baryon power spectrum (Pcb), while galaxy-galaxy lensing may require a combination such as sqrt(Pmm * Pcb).

🔄 Changes

• Add use_Pcb flag inside PositionTracer
• Propagate Pcb insisde AngularPowerSpectrum

🛠 How to Test

📝 Documentation

• This PR updates documentation
• This PR does not require documentation changes
• Issue created for documentation update: Don't forget to link the task!

🏗 Related Issues

Resolves #563

📸 Screenshots (if applicable)

📌 Additional Notes

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[gcanasherrera]

One question

[gcanasherrera]

Any chance we could jaxify this @matteobaratto @marcobonici ?