Add EER electricity demand profile

[WeiAi-Energy]

Closes #756.

Changes proposed in this Pull Request

• Add electricity.demand.profile: eer for power-demand builds.
• Retrieve the selected EER HDF5 demand dataset from Zenodo: https://zenodo.org/records/18435264.
• Add electricity.demand.scenario.eer_file to select one of:
  • demand_EER2025_100by2050.h5
  • demand_EER2025_Baseline_AEO2023.h5
  • demand_EER2025_IRAlow.h5
• Read state-level EER demand with the existing population-based state-to-bus disaggregation path and no AEO/EFS scaling.
• Validate EER configuration limits:
  • scenario.planning_horizons must use supported EER model years: 2021, 2025, 2030, 2035, 2040, 2045, 2050.
  • renewable_weather_years must contain exactly one supported EER weather year: 2007-2013 or 2016-2023.
• Align EER hourly CST (UTC-06:00) profiles to the existing naive UTC-style demand snapshot convention.

Validation performed:

• python -m py_compile workflow/scripts/build_demand.py workflow/scripts/retrieve_eer_data.py workflow/scripts/test/test_build_demand_eer.py
• C:\Users\23963\anaconda3\envs\pypsa-usa\python.exe -m pytest -q workflow/scripts/test/test_build_demand_eer.py
• Snakemake dry-run for all three EER retrieve targets:
  • data/eer/demand_EER2025_100by2050.h5
  • data/eer/demand_EER2025_Baseline_AEO2023.h5
  • data/eer/demand_EER2025_IRAlow.h5
• Snakemake dry-run for resources/Default/usa/demand/power_electricity.csv with local electricity.demand.profile: eer, confirming the configured EER file feeds build_electrical_demand.

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.