[WIP] Adopt `ghi_clear` variable name for clearsky GHI

[RDaxini]

• [X] Partially closes #2272
• [X] 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`).
• [X] 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.

Also related: #1253