gh-122: added alpha_s to cloelike

[ivansladoljev]

🚀 Pull Request Checklist

✅ Summary

Adds support for the running of the scalar spectral index (alpha_s = d n_s / d ln k) in CAMBBackground and propagates it through all cloelike likelihoods.

🔄 Changes

• EuclidLikelihood_photo_Cls.py, EuclidLikelihood_photo_2pcf.py: added "alpha_s" to the parameter key list forwarded to Background
• EuclidLikelihood_GCspectro_Pls.py, EuclidLikelihood_BAO.py: added alpha_s=parameters["alpha_s"] to Background(...) calls

🛠 How to Test

alpha_s defaults to 0.0 so all existing sampling scripts remain valid without modification. Scripts that want to vary it simply need to include alpha_s in their parameter dictionary and prior.

📝 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 CAMBBackground does not support running of the spectral index (alpha_s) #122

📌 Additional Notes

Complements the similar PR in cloelib

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[gcanasherrera]

Hi @ivansladoljev I was coming back to this. Could you solve the conflicts?

[gcanasherrera]

@raphkou when @ivansladoljev fixes the conflicts, you can also proceed with this one.