[FEATURE] Improve `apply_reference_operation` Compatibility with eGon Project Data

[khelfen]

Motivation / Problem

apply_reference_operation
does not work out of the box with home-battery data from the eGon project, and there is no dedicated
method on the EDisGo class to call it directly. This makes integrating it into eGon-data workflows
cumbersome and error-prone.

Proposed solution

• Make apply_reference_operation work with eGon project data without extensive preprocessing:
  • Set default values for battery efficiency when none are given
  • Add clearer error messages — e.g. when generators were not loaded before the call, or when time series for generators/loads were not set
• Add a dedicated method on the EDisGo class to call apply_reference_operation directly

Alternatives considered

• Manually preprocessing the eGon data before calling the function — adds complexity and reduces usability.

Affected files

• edisgo/flex_opt/battery_storage_operation.py (apply_reference_operation)
• edisgo/edisgo.py (new EDisGo method)

Acceptance criteria

• apply_reference_operation runs on eGon home-battery data without manual preprocessing
• Sensible default battery efficiency when unspecified
• Actionable error messages for the missing-generators / missing-time-series cases
• A documented EDisGo method exists, with a test using eGon-style input