feat: termination condition map for SCIP solver

[siddharth-krishna]

Closes # (if applicable).

Changes proposed in this Pull Request

Currently, if the user sets the limits/gap SCIP solver option to stop solving MIPs when a certain duality gap is reached, then linopy reports the termination condition as unknown. This PR tries to map the SCIP statuses to linopy termination conditions, using
https://github.com/scipopt/scip/blob/b2bac412222296ff2b7f2347bb77d5fc4e05a2a1/src/scip/type_stat.h#L40

(CC @pfetsch, I couldn't find the exact mapping from the enum integers to the strings returned by pyscipopt so I guessed that it's the same as the names of the enums)

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.

[pfetsch]

The mapping looks good, except possibly for gap limit reached. This should probably be handled differently. Currently, you cannot determine whether this happened or some other limit was reached. I think that for the other solvers, a gap limit reached is treated as optimal.

[siddharth-krishna]

Thank you @pfetsch ! That makes sense, I've opened a new PR to fix that: #448