[grlee77]

Description

This PR removes the need for the channel_as_last_axis decorator utility for functions in the skimage.filters module. This decorator allowed existing functions that assumed channels along the last axis to work without modification. However, ideally we will replace this decorator with native support for arbitrary channel position in each function (with possible exception of cases where it would substantially increase the code complexity).

This PR does not introduce any change to existing behavior. Some new tests involving channel_axis were added to an existing difference_of_gaussian test function.

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
• Descriptive commit messages (see below)

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.

[rfezzani]

Thank you @grlee77 😉