fix load weighted prices

[lisazeyen]

Changes proposed in this Pull Request

Fix bug when calculating load weighted prices in the make_summary script

Checklist

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