fix: call GLPK with --freemps instead of --mps

[siddharth-krishna]

Closes #402

Changes proposed in this Pull Request

GLPK solver is always called with the --freemps option instead of the --mps when using the Solver API to solve an external MPS file. --mps is for the older fixed-column MPS format that is rarely used nowadays. Almost all fixed MPS files can be parsed by the free MPS format.

As @odow says in #402:

Just always pass --freemps. Pretty much any fixed format MPS file should be parseable as freemps. The only issue might be if there are variable or column names with leading spaces. But JuMP would never write one, and I don't think Linopy would. And anyone that does is cursed.

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.