[WIP] Refactor histogram to separate logic for multi- and single-channel images

[maaleske]

Description

In preparation for future histogram options for multichannel images, this PR separates the single-channel histogram calculation to an internal function, with any channel-number dependent branching (currently only the warning) in the exposed function.

Checklist

• No new functionality
• Rebase on ENH: add range option for histogram. #2479

References

• The changes in ENH: add range option for histogram. #2479 probably need to be taken into account
• This enables cleaner branching for implementing [WIP] Optional multichannel histograms  #2788

For reviewers

(Don't remove the checklist below.)

• 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.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a665476). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2786   +/-   ##
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  Coverage          ?   85.42%           
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  Files             ?      330           
  Lines             ?    26483           
  Branches          ?        0           
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  Hits              ?    22624           
  Misses            ?     3859           
  Partials          ?        0

Impacted Files	Coverage Δ
skimage/exposure/exposure.py	100% <100%> (ø)

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