Replaced stale charging_park_id with the drawn_charging_park_id in th…

[jldonner]

Description

Used the correct variable

drawn_charging_park_id 

in the home branch's capacity bookkeeping:

designated_charging_point_capacity_df.at[  
    drawn_charging_park_id, "designated_charging_point_capacity"  
] += charging_capacity

Fixes #679

Type of change

Please delete options that are not relevant.

• 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 fix is right. The stale id always belongs to a work park, so the increment
lands outside the slice combine_weights reads for the home parks — the capacity column
stays all zero and the feedback term degenerates into a constant. Assignment of home
cars changes measurably once there is contention between parks.

Two things before merge:

The NOTE above the loop and the inline comment above the fixed line both still say the
stale id is intentional and point at #679. Please drop them, they now describe the
opposite of what the code does.

whatsnew entry, and a regression test. Note test_distribute_charging_demand cannot
catch this — it checks totals per park, and those were always right because
designated_charging_point_capacity is regrouped from charging_processes_df. Something
like patching combine_weights and asserting the second car sees a nonzero entry at the
first car's park would hit the actual mechanism.