feat: move set_int_index to solution assignment in model.py

[FabianHofmann]

Follow up on #396

The internal handling of Solution objects was improved for more consistency. Solution objects created from solver calls now preserve the exact index names from the input file.

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.

[siddharth-krishna]

Thanks Fabian, this works for me! If I understand the idea correctly, I think it's also a more elegant approach, and ensures we only call set_int_index on solutions returned from linopy.Model, which is guaranteed to have the form x1, x2, ....

Btw, here is a test file I have been using to test this issue. If it makes sense, I can add it to the tests in this PR, if you can point me to where/how to integrate them. Thanks!

#!/usr/bin/env python3
"""
Created on Tue Jan 28 09:03:35 2025.

@author: sid
"""

import sys
from pathlib import Path
from linopy import solvers

mps_problem = """
NAME        sample_mip
ROWS
 N  obj     
 G  c1      
 L  c2      
 E  c3      
COLUMNS
    col1        obj       5
    col1        c1        2
    col1        c2        4
    col1        c3        1
    MARK0000  'MARKER'                 'INTORG'
    colu2        obj       3
    colu2        c1        3
    colu2        c2        2
    colu2        c3        1
    col3        obj       7
    col3        c1        4
    col3        c2        3
    col3        c3        1
    MARK0001  'MARKER'                 'INTEND'
RHS
    RHS_V     c1        12
    RHS_V     c2        15
    RHS_V     c3        6
BOUNDS
 UP BOUND     col1        4
 UI BOUND     colu2        3
 UI BOUND     col3        5
ENDATA
"""
input_file = "/tmp/sid-linopy.mps"
with open(input_file, "w") as f:
    f.write(mps_problem)

s = solvers.Highs()
result = s.solve_problem(problem_fn=Path(input_file), solution_fn=Path("/tmp/sid-linopy-out.sol"))
print(result)
print(result.solution.objective)

[FabianHofmann]

great, since this a new class of test (which only tackles the solver implementations) I would suggest we create a new module test/test_solvers.py where we test these low-level solver routines. what do you think?

[FabianHofmann]

you can push it directly to this PR (permissions should be in place)

[siddharth-krishna]

Thanks, Fabian! I added a new test file. I also added some common virtualenv paths to the .gitignore, if you don't mind?

Looks like the test is failing on some solvers: glpk, cbc

Do you think it's worth investigating, or should I reduce the scope of the test to HiGHS?