Deprecate unit_cube-related methods in Axes3D

[oscargus]

PR Summary

I cannot see that there is any valid public use for these. No doc-string either.

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).

[tacaswell]

Maybe soften this to "...if you rely on this let us know and we will consider preserving"?

[oscargus]

Maybe soften this to "...if you rely on this let us know and we will consider preserving"?

Doesn't that always hold?

(I checked on grep.app and one other library seems to use tunit_edges.)

[timhoffm]

Doesn't that always hold?

Basically, yes. But people may not be aware of that. However, IMHO adding this is is only needed in cases where we ourselves have some doubts and consider it likely that there might be substantial downstream issues.

This here is good as is.

[timhoffm]

@oscargus maybe you want to give the one known downstream user a heads up.