Closes #1493

Changes proposed in this Pull Request

This PR fixes a bug in PyPSA ≥ 1.0 where ramp constraints were applied to fixed generators and links with NaN ramp_limit_up/down once ramping was enabled for any other asset of the same component.

In the define_ramp_limit_constraints function, the masking logic for fixed components currently uses:

~ramp_limit_up_fix.isnull().all()

Calling .all() collapses the array across all dimensions and produces a scalar boolean. As a result, once any fixed asset has a ramp limit defined, the ramp constraint is applied to all fixed assets, including those with NaN ramp limits.

Consequently, assets without specified ramp limits can receive ramp constraints equal to zero, leading to unintended infeasibilities (e.g. for load-shedding or slack generators).

This PR replaces the scalar check with element-wise masking, so that ramp constraints are only created for assets (and snapshots) where ramp limits are explicitly defined.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.