Have pvlib.iotools.read_solrad return metadata

[AdamRJensen]

• [ ] Closes #xxxx
• 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.

The read_solrad function currently does not follow the standard pattern of returning (data, metadata). This PR adds metadata parsing to the function and is thus a breaking change.

[kandersolar]

I've been persuaded that this is minor enough to be more of an enhancement than a breaking change, and so should go in 0.10.4. I changed the milestone accordingly.

Nice improvements to the tests by the way!