add transformer to __init__.py

[kurt-rhee]

• Closes None
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• 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.
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Hello maintainers this is a single line commit that adds the transformer to the init.py file in the pvlib folder. This will allow people to use pvlib.transformer.simple_efficiency instead of having to use from pvlib.tranformer import simple_efficiency.

[kandersolar]

Good find @kurt-rhee. Can you add a whatsnew entry? I think this could be called a bugfix.