Extend resource-specific functionality for geographic transmission

[espenbodal]

Description

This issue implements resource-specific extension functionality for EnergyModelsGeography, following the pattern established in EnergyModelsBase (see EnergyModelsBase#61).

Motivation

Building on the resource-extension framework in EMB, this feature enables EnergyModelsGeography users to define custom resource types with geography-specific variables and coupling constraints. This allows modeling of resource-specific properties (e.g., potential power, temperature in district heating, pressure/composition in gas networks) that degrade or transform during transmission across areas.

Feature Overview

• New methods: variables_flow_resource and constraints_couple_resource for transmission modes and areas
• Use case: Track multi-attribute resources (e.g., potential, temperature, pressure) alongside standard flows
• Coupling: Enforce area-level mass balance with transmission-level resource tracking

Related Issues & Topics

• Related to: EnergyModelsBase#61 (resource-extension framework)
• Development branch: enhance/complex_resource_flow

Documentation

Comprehensive how-to guide added at docs/src/how-to/extend-resource-functionality.md with concrete PotentialPower example demonstrating:

1. Custom resource type definition
2. Resource-specific variable creation
3. Transmission mode constraint implementation
4. Area-to-transmission coupling constraints

Tests

Reference implementation and comprehensive tests in test/test_resource_flow.jl including:

• PotentialPower resource definition and mode implementation
• Variable verification and constraint satisfaction checks
• Integration tests with full optimization

Example Applications

The extension pattern is useful for modeling:

• Power flow (reactive power, voltages, voltage angles etc.)
• District heating networks (temperature tracking)
• Natural gas networks (pressure and gas mixture composition)
• Any carrier with quality degradation or state attributes