fix: don't disable presolve and fix dual value retrieval for SCIP

[siddharth-krishna]

Closes # (if applicable).

Changes proposed in this Pull Request

Feedback from SCIP developers in open-energy-transition/solver-benchmark#148 indicates that linopy might be making an error when retrieving dual solution values from SCIP. Also, they recommend against disabling presolve, which might have been done in order to retrieve dual solution values. This PR makes both changes.

I think we might not need a separate function for LPs and MILPs, since the dual retrieval code is in an if-then block?

We should also test that removing the code that filters dual variable names to only those starting with c or cf is no longer necessary.

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.