Add heating curve

[benspoek]

Is your feature request related to a problem? Please describe.

• Supply and return temperatures are currently not available in the demand forecasting tool.
• COP calculations for heat pumps therefore lack temperature-level consistency.
• District heating configurations (e.g., different temperature regimes) cannot be compared properly.

Describe the solution you'd like

• Add supply temperature as an output feature.
• Derive supply temperature from a linear heating curve f(outdoor temperature, zone temperature).
• Set heating curve automatically based on building year and retrofit status.
• Calculate return temperature from current heat demand assuming fixed mass flow.

Describe alternatives you've considered

• Implement a physically meaningful ΔT logic between supply and return temperature (e.g., characteristic temperature spread depending on building type).

Additional context

• Required for consistent COP estimation of heat pumps.
• Enables comparison of district heating temperature levels.
• Relevant for retrofit assessment and network configuration studies.