Improve func name, example, directory management

Function name has been changed to `make_persistent` after prompt by @lemonsaurus
asking for a more descriptive name. Thanks @MarkKoz for providing the alternate
name.

During local testing, the `data` directory doesn't exist yet. In prod, this
isn't an issue as the persistent volume is mounted at that location. To make
local testing more convenient, the directory is checked and made if not found.

Persistent data files will be placed in a seasonal subdirectory so long as they
have a valid season name somewhere in their path, otherwise they will be placed
directly in the data directory.

Added a note to docstring to avoid same-named files in the same seasons or it
will conflict with each other in the persistent data directory.

The example was extended a little bit to make it both actually valid if tested
and hopefully make it easier to understand what's going on.

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