Fix OPF time_elapsed calculation

[jldonner]

Description

This PR fixes the calculation of time_elapsed in edisgo/io/powermodels_io.py when converting an eDisGo/PyPSA network to the PowerModels format.

Previously, time_elapsed was derived using .seconds and cast to int:

int((psa_net.snapshots[1] - psa_net.snapshots[0]).seconds / 3600)

This caused incorrect values for several valid snapshot resolutions:

15-minute snapshots resulted in time_elapsed = 0 instead of 0.25
24-hour gaps resulted in time_elapsed = 0
25-hour gaps resulted in time_elapsed = 1 instead of 25

Since time_elapsed is used in the Julia OPF formulation for inter-temporal constraints, this could decouple power and energy variables for batteries, heat storage, DSM, and EV charging.

The fix introduces an internal helper function _get_time_elapsed_in_hours(snapshots) which:

1. uses .total_seconds() instead of .seconds
2. keeps the result as a float instead of casting it to int
3. checks that the snapshots are equidistant
4. raises an error for invalid or non-equidistant snapshot indices

Fixes #693

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them the setup.py, and if needed the rtd_requirements.txt, the eDisGo_env.yml and the eDisGo_env_dev.yml.
• I have added new features to the corresponding whatsnew file

[joda9]

@jldonner Can you write a test for this function?