fix/scale vom by efficiency when missing

[gianvicolux]

Closes #2145.

Changes proposed in this Pull Request

This PR fixes a bug concerning marginal cost definition for those links where vom costs are given per unit of output (e.g., electricity) in the technology cost datasets. Indeed, marginal_cost of links must be expressed per unit of carrier at bus0, because the PyPSA objective function multiplies marginal_cost × p0 × snapshot_weighting. In such cases, the correct conversion when setting marginal_cost for a Link is:

marginal_cost [EUR/MWh_bus0] = efficiency [MWh_busN/MWh_bus0] × VOM [EUR/MWh_busN]

In multiple places across prepare_sector_network.py and add_existing_baseyear.py, marginal_cost was set directly to costs.at[tech, "VOM"] without multiplying by efficiency. This caused an overestimation of the effective marginal cost by a factor of 1/η.

The affected carrier were:

• central solid biomass CHP, central solid biomass CHP CC
• CCGT methanol, CCGT methanol CC
• H2 turbine
• urban central {fuel} CHP, urban central {fuel} CHP CC
• biogas to gas, biogas to gas CC

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.rst.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.rst files.