Add __name__ to Halofit partial models for logging

[rrjbca]

Description

Add name to halofit partial models (halofit_smith, halofit_takahashi, halofit_bird) as this is required for pipeline logging. Partial fix to documentation build error in the power spectrum example plot.

~N.b. the readthedocs build error AttributeError: 'function' object has no attribute 'Om0' is a separate issue addressed by #527. This PR should be merged after that one.~

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request