fix: treat SCIP condition gaplimit as optimal

[siddharth-krishna]

Changes proposed in this Pull Request

As a SCIP developer pointed out in #447 (comment) the "gap limit reached" condition should be treated as an optimal termination condition.

Checklist

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