multichannel to channel_axis (2 of 6): transform functions

[grlee77]

follows #5284. implements channel_axis in the transform module

Description

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[pep8speaks]

Hello @grlee77! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-01 11:56:17 UTC

[jni]

@grlee77 I've made only a very minor comment, which I should have caught in the last PR — shouldn't some tests be updated to use the new form rather than wrapped in an expected_warnings context?

[grlee77]

This one has one review and should be good to go if anyone else could do a quick check

[alexdesiqueira]

This one is huge! Thank you Greg!