finishing issue-1188

[MichaelHopwood]

• Closes change eta_m to module_efficiency #1188
• 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.

We conducted a global search and replaced all reference to eta_m and eta_m_ref with module_efficiency. However, we are not quite sure how to incorporate the deprecation cycle, as referred to in the original ticket. Additional guidance for this process will be appreciated.

[cwhanse]

@MichaelHopwood see #1218 maybe you could look at #1118?