Adjusted the time exponent used in the heat storage energy constraint…

[jldonner]

… for inter-time-step state-of-charge coupling. Now fractions of hours can be simulated.

Description

The equation of the constraint_store_state() function no has the elapsed time now in the exponent making fractions of hours possible to propperly calculate.

Fixes #697

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