gh-510: Method sigma8_0 failing on repeated calls for HMemu perturbations

[NastassiaG]

🚀 Pull Request Checklist

✅ Summary

This PR modifies the self.sigma8_0() method in HMemuNonLinearPerturbations to avoid failure on repeated calls. It is also adding the same method to HMemuLinearPerturbations, where it is currently missing.

🔄 Changes

• HMemuNonLinearPerturbations: Renaming the instance attribute self.sigma8_0 assigned inside the method self.sigma8_0() to self.sigma8; this avoids the method being shadowed by an array (causing failure on repeated calls).
• HMemuLinearPerturbations: Implementing the missing self.sigma8_0() method by reusing the updated implementation from the nonlinear perturbations class.

🛠 How to Test

1. Create instances of both HMemuLinearPerturbations and HMemuNonLinearPerturbations.
2. Verify that the sigma8_0() method exists for both instances.
3. Verify that, upon repeated calls, it yields the expected result with no exceptions raised.

See also screenshots below.

📝 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 #510

📸 Screenshots (if applicable)

Before implementing changes:

After implementing changes:

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[gcanasherrera]

Hi @NastassiaG,

I was unable to reproduce your original reported bug, really. I've been running chains with this implementation for months now. Before we proceed with the review, could you specify a bit more what the issue could potentially be?