In fs mode, replace KEY positional arguments with --key-path

[maddenp-noaa]

Description

Currently the fs actions copy, link, and makedirs take an arbitrary number of positional KEY arguments on the CLI, and an optional keys API argument, to specify a keypath leading to the config block to use. This should be harmonized with the rest of uwtools to use CLI --key-path and API key_path, with matching semantics.

AC

• CLI is updated to use --key-path.
• API is updated to use key_path.
• Documentation is updated.
• Unit tests are updated.
• Jupyter notebooks are updated (if needed).