fix ForecastModel.get_data handling of timezones

[wholmgren]

• Closes Apparent issue with timestamp handling & solar position calculations #1237
• I am familiar with the contributing guidelines
• Tests added I haven't used this code in years and don't care to add tests for it. "highly experimental" indeed
• 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 hope this is the last time I ever open Pandora's box.

[kandersolar]

Not hitting "approve" here because I can't do proper reviews for the forecast module, but this does seem to fix my test case from #1237 👍

[wholmgren]

because I can't do proper reviews for the forecast module

I don't think anyone can at this point, including myself. Another reason for #1057, but let's kick that into the next release cycle.