Modify demand_limiting_capacity constraint

[tsmbland]

Description

This modifies the demand-limiting capacity constraint to address the issue described in #457. I've added lots of comments to the code so hopefully everything is relatively clear, but happy to have a chat about it if not.

None of the example models are affected, as this only affects certain technologies like refineries that have multiple end-use commodities, but I've tested this with Alaa's Qatar model and the results look reasonable.

Fixes #457

Type of change

Please add a line in the relevant section of
CHANGELOG.md to
document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[dalonsoa]

Looks good but very hard to understand the underlying logic - specially in the long term. I suggest you extract the new code as a standalone function and add some dedicated test - can even be a doctest, so it is included with the documentation - illustrating what is going on here and what problem is solving.

[tsmbland]

@dalonsoa I've extracted the code to a new function and created a doctest, as suggested, so hopefully it's clearer now.

Also, things started to fail with python 3.12 due to the numpy 2.1 release, so I've had to pin numpy to 2.0 for now.

[dalonsoa]

I wish all MUSE code had this detailed explanations...