Add `adapt_rgb` decorator and helpers.

This applies `adapt_rgb` to `equalize_adapthist` and removes the
special-casing of RGB images in that function. Note that tests of
`adapt_rgb` fail because some type conversion and intensity scaling
were added to pass tests for `equalize_adapthist`.
Remove cast and rescale and adjust test values to match
Fix docstring for wrapped functions
Remove orphaned import
Merge pull request #1073 from tonysyu/feature/adapt_rgb

Add `adapt_rgb` decorator and helpers