Small fixes for cobaya wrapper

[lukashergt]

Minor fixes for running with Cobaya.

Checklist:

• I have performed a self-review of my own code.
• My code is PEP8 compliant (flake8 --max-line-length 99 primpy tests).
• My code contains compliant docstrings (pydocstyle --convention=numpy primpy).
• New and existing unit tests pass locally with my changes (python -m pytest).
• I have added tests that prove my fix is effective or that my feature works.
• I have appropriately incremented the semantic version number in both README.rst and primpy/__version__.py.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.09%. Comparing base (7fb3614) to head (a5432dd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #75   +/-   ##
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  Coverage   99.09%   99.09%           
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  Files          19       19           
  Lines        2439     2439           
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  Hits         2417     2417           
  Misses         22       22           

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[Copilot]

Pull request overview

This PR makes minor compatibility fixes for running primpy with the Cobaya wrapper, including forwarding additional configuration to PPS derivation and small metadata/version updates.

Changes:

• Allow calibrate_scale_factor to accept and forward extra kwargs to derive_approx_power.
• Improve Cobaya slow-roll PPS wrapper robustness (import fallback, expose DeltaN_minus1, handle rho_reh_GeV=None).
• Update potential display labels and bump version to 2.18.3.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
primpy/potentials.py	Adjusts TeX labels for Double-Well potentials and changes TmodelPotential.tag.
primpy/inflation.py	Adds **kwargs to calibrate_scale_factor and forwards them to derive_approx_power.
primpy/__version__.py	Bumps library version to 2.18.3.
cobaya_wrapper/slowroll_pps.py	Adds import fallback, exposes DeltaN_minus1, and avoids None comparison for rho_reh_GeV.
README.rst	Updates displayed version to 2.18.3.

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