During color conversion, maintain float precision

[hmaarrfk]

Description

Color conversion currently imposes 64 bit types. I don't think this should be the case. Many images are taken with 8bit sensors and as such 32 bit floating point types are probably adequate.

I would rather actually fix this in img_as_float itself. See the PR #3062.

Checklist

[It's fine to submit PRs which are a work in progress! But before they are merged, all PRs should provide:]

• Clean style in the spirit of PEP8
• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Unit tests

[For detailed information on these and other aspects see scikit-image contribution guidelines]

References

[If this is a bug-fix or enhancement, it closes issue # ]
[If this is a new feature, it implements the following paper: ]

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

Merging #3060 into master will decrease coverage by <.01%.
The diff coverage is 90%.

@@            Coverage Diff            @@
##           master   #3060      +/-   ##
=========================================
- Coverage    85.9%   85.9%   -0.01%     
=========================================
  Files         336     336              
  Lines       27301   27312      +11     
=========================================
+ Hits        23454   23463       +9     
- Misses       3847    3849       +2

Impacted Files	Coverage Δ
skimage/util/tests/test_dtype.py	100% <100%> (ø)	⬆️
skimage/color/colorconv.py	98.55% <88.88%> (-0.56%)	⬇️

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