PSM3 updates

[kandersolar]

• Closes Adjustable attributes for get_psm3 #1079
• I am familiar with the contributing guidelines
• Tests added
• Updates entries to docs/sphinx/source/api.rst 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 and Milestone are assigned to the Pull Request and linked Issue.

I'm lumping a few get_psm3 updates into this PR:

• Add optional attributes parameter to address Adjustable attributes for get_psm3 #1079
• Update list of acceptable years in docstring
• Add support for the new 5-minute and 15-minute API endpoint

I'm not wild about including a 10MB csv file for the test -- okay to trim it down to a single day, or fine as is?

[wholmgren]

I'd prefer to cut the csv down to just a day. I've done that for a lot of the other test files.