BUG/STY: update _get_file in download

[jklenzing]

Description

The get_file helper function was added to support downloading files that may need to be unzipped before local archival. This cleans up several issues identified in the RC. No update to changelog as this is captured by other changes in this version.

• Renames function to _get_file since this is not intended to be called directly by a user.
• Adds a ValueError if this should be invoked incorrectly.
• Adds test for the new error.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

pytest pysatNASA/tests/test_methods_cdaweb.py

Test Configuration:

• Operating system: Mac OS X Sonoma
• Version number: Python 3.12

Checklist:

• Make sure you are merging into the develop (not main) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes
• Update zenodo.json file for new code contributors

If this is a release PR, replace the first item of the above checklist with the release
checklist on the wiki: https://github.com/pysat/pysat/wiki/Checklist-for-Release