Refactor non-blocking download in Subsetter class for clarity

- Updated the _non_blocking_download method to create individual argument lists for each parameter instead of using zip. This improves readability and maintainability of the code.
- Simplified the argument passing to the process_map function by explicitly defining the lists for URLs, local paths, and overwrite flags.

Also, cleaned up import statements in test_kernels.py for consistency.

Author: michaelaye