Correction for 2024 electricity demand

[AlessandroPampado99]

If you attempt to run PyPSA-Eur using the 2024 weather year, the electricity demand script fails because no demand data are available for that year.
A workaround is to set fixed_year: 2019 (or lower), but this still requires correcting how synthetic demand data are integrated — they should refer to the fixed year rather than to snapshots, and the resulting load must be reindexed properly.

Changes proposed in this Pull Request

• Reindex correctly load to the snapshots to avoide Nan values
• Refer the synthetic data to years instead of snapshots (more robust behaviour)

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.

[fneum]

Will be fixed in #1828.

[fneum]

Closing in favour of #1828