fix: remove key_prefix from time features

[anxkhn]

Pull Request

Description

This pull request removes the key_prefix argument from the make_datetime_numpy_dict and make_sun_position_numpy_sample functions and updates all call sites and tests accordingly. The key_prefix was originally introduced for flexibility in naming output dictionary keys, primarily within the context of the now-deprecated ocf_datapipes library. Since ocf-data-sampler uses a different approach (PyTorch Datasets instead of DataPipes), this flexibility is no longer needed and introduces unnecessary complexity.

This PR simplifies the code by hardcoding consistent key names for the datetime and sun position features:

• make_datetime_numpy_dict: Keys are now "date_sin", "date_cos", "time_sin", "time_cos".
• make_sun_position_numpy_sample: Keys are now "solar_azimuth", "solar_elevation".

This change improves code readability and maintainability by removing a parameter that is no longer relevant and ensuring consistent naming across the codebase.

Fixes #159

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (No docstring changes were necessary, as the functions' behavior didn't fundamentally change, only the output keys)
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[Sukh-P]

Thanks for making a start on this @anxkhn, I will assign you to the issue so there is no confusion on who is working on this, I think some of the tests still need updating as the expect the key prefix still and there are some merge conflicts but hopefully should be good to go after that!

[anxkhn]

Hi, I will work on resolving tests soon.