perf: use xpress c interface, xpress solution order of operations improvement, remove writing of redundant solution files

[CharlieFModo]

Closes # (if applicable).

Changes proposed in this Pull Request

• performance improvements to Xpress._solve_problem_from_file
• use the solver's C interface directly to extract constraint and variable names rather than list comprehension
  • casting to string the xpress objects returned by problem.getConstraints() etc. is expensive and so is accessing the names via .name.
  • this gives small performance improvement on larger models
• improve order of operations when extracting dual
  • for MILPs, let code hit the exception on getDuals before doing the expensive constraint name extraction
• use writebinsol for a c. 10x improvement from writesol
  • @FabianHofmann it would be nice to not write the solution at all for xpress if keep_files=False

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.

[FabianHofmann]

that was fast ;)

[FabianHofmann]

@FabianHofmann it would be nice to not write the solution at all for xpress if keep_files=False

I very much like that, and I honestly have the feeling that this is a redundant artefact from an older implementation. So I would also be happy to remove the writing of the sol file entirely if not needed by us.

[CharlieFModo]

I've pushed a small change that should remove the writing of the solution file in situations where it isn't needed or requested. For now, I've left the concept in there in case people are relying on the keep_files=True solve option

[FabianHofmann]

I would like to avoid this hard-coded list of solvers here, rather define is in the "preamble" of solvers.py and import from there or add the logic to the solver classes (would however mean to pass keep_files to them)

[CharlieFModo]

Thanks! Have done the first option for a smaller set of changes. I originally thought about passing the keep_files parameter through to the solvers but it felt a little messy.

[FabianHofmann]

great @CharlieFModo