Closes # (if applicable).

Add a vent for district heating

This PR should add a vent for district heating. Currently, with the must run condition for Fischer-Tropsch synthesis, waste heat usage enabled and decreasing heating demand (as assumed exogenously in the config default setting), pathway optimisation leads to infeasibilities in some investment periods since large Fischer-Tropsch capacities are needed in early years but the waste heat exceeds the heat demand in later investment periods.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.