Adding functions for retrieving NSRDB PSM4 Polar datasets

[etoni044]

Added two new functions to psm4.py: get_nsrdb_psm4_polar and get_nsrdb_psm4_polar_tmy using the methodology created for other psm4 datasets.

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I'm not entirely sure how to proceed with this pull request, as this is my first time going through the process.

Checklist:

• Closes Add NSRDB polar datasets #2639
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

[kandersolar]

Thanks @etoni044, the functions look good on first glance. The next step for this PR will be to add tests. #2378 is an example you can follow. And here is some general info about testing: https://pvlib-python.readthedocs.io/en/stable/contributing/testing.html

If you get stuck, feel free to update this PR with whatever code you have and someone will come help!