Numpy >2.0 bug for indexing in utils multiply()

[dalyw]

#54 - one solution, Alex had some ideas too on pyomo usability

For numpy ≥ 2.0, the numpy charge_array was indexed by the Pyomo time-set member instead of its position. Worked when model.t = range(T), but raised IndexError for model.t = any other ordered set (e.g. float seconds).

Debugged using Claude: Added helper at(expr, t). Numpy operands are indexed by their ordinal position, Pyomo operands by their member.

Added a non-range test case to test_multiply_pyo.

Pull request recommendations:

• Name your pull request your-development-type/short-description. Ex: feature/gui
• Link to any relevant issue in the PR description. Ex: Resolves [ENH: Support optimization of flexibility metrics #12]
• Provide context of changes.
• Provide relevant tests for your feature or bug fix.
• Provide or update documentation for any feature added by your pull request.

Thanks for contributing!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.37%. Comparing base (d044d34) to head (b642fcf).

Files with missing lines	Patch %	Lines
eeco/utils.py	83.33%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (94.11%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   94.38%   94.37%   -0.01%     
==========================================
  Files          10       10              
  Lines        1603     1601       -2     
==========================================
- Hits         1513     1511       -2     
  Misses         90       90              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.