Solver Refactor Part 2: Reduce Duplication & Restructure for Future Redesign

[mrmundt]

Fixes #1030 (partly)

Summary/Motivation:

This does not fully complete the solver refactor (though let's be honest, it'll never really be "done") and builds on the work introduced in #3137. This is a "save state", however, that introduces HiGHs into the new solver interfaces as well as introduces necessary restructuring for clarity and future changes.

There is minimal functionality change for users as a result of this PR. The main new things they will see are:

• Access to highs_v2
• Rename gurobi_v2 to gurobi_persistent_v2
• Rename Results.solver_configuration to Results.solver_config

Changes proposed in this PR:

• Introduce highs_v2 interface
• Restructure directory in contrib/solver to have common, solvers, and tests
• Renaming of certain modules for clarity (e.g., solution -> solution_loader)
• Reduce duplicate code by creating mixin classes as applicable
• Future-port assert_optimal_termination and check_optimal_termination to make it forwards and backwards compatible
• Clean up legacy mapping logic
• Generally apply pylint suggestions (because static analysis is good!)

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[mrmundt]

Temporarily closing because I didn't realize Windows was being problematic...