Do not allow EmissionPenalty to be used as subsidy for negative emissions

[willu47]

This resolves the issue identified in issue #99 by @Timon-R, where an EmissionPenalty acts as a subsidy for negative emission technologies, providing a double-dividend. E.g. the technology receives a benefit once for avoiding positive emissions, and then receives a subsidy for the negative emissions.

Closes #99.

[Copilot]

Pull Request Overview

This PR ensures that emission penalties are no longer applied (or subsidized) when EmissionActivityRatio is negative, preventing a double dividend for negative-emission technologies.

• Constrain EmissionsPenalty to be non-negative
• Update E5 (and E3) constraints to sum only positive EmissionActivityRatio entries
• Remove non-negativity on emission variables to allow tracking negative emissions

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/test_negative_emissions.txt	New test scenario data for negative emissions subsidy behavior
src/osemosys_short.txt	Add EmissionsPenalty >=0 and filter E5 sum to >0 ratios
src/osemosys_fast.txt	Filter E5 sum to only positive ratios in fast model
src/osemosys.txt	Add non-negativity on EmissionsPenalty, allow negative vars, update E5 & E3 sums, clean positivity constraints

Comments suppressed due to low confidence (5)

src/osemosys.txt:1061

• This filter excludes negative emissions from the AnnualEmissionsResults table; change the condition to 'AnnualEmissions[r, e, y] <> 0' to include negative emission entries.

AnnualEmissions[r, e, y] > 0

src/osemosys.txt:1076

• This filter omits negative emission values in the AnnualTechnologyEmissionResults table; use '<> 0' instead of '> 0' to capture negative emissions.

AnnualTechnologyEmission[r, t, e, y] > 0

src/osemosys.txt:1083

• The table filter here drops negative emission entries; consider using 'AnnualTechnologyEmissionByMode[r, t, e, m, y] <> 0' to include negative values.

AnnualTechnologyEmissionByMode[r, t, e, m, y] > 0

src/osemosys.txt:309

• [nitpick] Consider adding a comment to explain that this variable can be negative to represent negative emissions, improving clarity for future maintainers.

var AnnualTechnologyEmissionByMode{r in REGION, t in TECHNOLOGY, e in EMISSION, m in MODE_OF_OPERATION, y in YEAR};

tests/test_negative_emissions.txt:42

• Add an explicit assertion or expected outcome in this test to verify that DiscountedTechnologyEmissionsPenalty is zero for technologies with negative EmissionActivityRatio.

BB gas_plant CO2 1 2016 -0.516