Refactor `med_term_purchasing` to fix benchmark issues

[tristantc]

Description

This Issue aims to address and refactor the med_term_purchasing model to fix benchmark issues as outlined in Issue #60.

Solution strategy causing issues:

• Big-M MINLP reformulation
• Hull MINLP reformulation
• GLOA
• Enumeration
• LOA
• LBB
• RIC

Branch

This PR targets the benchmark_zd branch.

Steps to Reproduce

1. Run the benchmark tests for the med_term_purchasing model before applying the changes.
2. Apply the changes from this PR.
3. Re-run the benchmark tests to verify performance improvements.

Notes

• Ensure that all sub-solvers are tested thoroughly after refactoring.
• Additional optimizations may be required based on benchmark results.

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Feel free to adjust any details as necessary before submitting the PR.