Refactor enhancements

[pnuu]

This PR continues with the tasks listed in https://github.com/pytroll/satpy/issues/3123 to split __init__.py of subpackages to modules by refactoring enhancements.

• [x] Tests added
• [x] Fully documented

[codecov[bot]]

Codecov Report

:x: Patch coverage is 99.39668% with 4 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 96.32%. Comparing base (1670af4) to head (1b3c974). :warning: Report is 212 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/enhancements/__init__.py	88.88%	1 Missing :warning:
satpy/enhancements/colormap.py	98.70%	1 Missing :warning:
satpy/enhancements/contrast.py	98.70%	1 Missing :warning:
satpy/tests/enhancement_tests/test_wrappers.py	98.11%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3155    +/-   ##
========================================
  Coverage   96.31%   96.32%            
========================================
  Files         454      465    +11     
  Lines       58045    58159   +114     
========================================
+ Hits        55907    56020   +113     
- Misses       2138     2139     +1     

Flag	Coverage Δ
behaviourtests	3.64% <15.38%> (+0.04%)	:arrow_up:
unittests	96.41% <99.39%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[coveralls]

Pull Request Test Coverage Report for Build 16820250408

Details

• 672 of 677 (99.26%) changed or added relevant lines in 17 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 96.422%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/enhancements/init.py	9	10	90.0%
satpy/enhancements/colormap.py	77	78	98.72%
satpy/tests/enhancement_tests/test_wrappers.py	53	54	98.15%
satpy/enhancements/contrast.py	78	80	97.5%
<!--	Total:	672	677

Totals
Change from base Build 16807860351:	0.005%
Covered Lines:	56347
Relevant Lines:	58438

---

💛 - Coveralls

[pnuu]

No warnings from the doc build with the -n in Makefile.

[pnuu]

I renamed stretching.py to contrast.py and color_mapping.py to colormap.py, so they have more consistent naming with the other modules.

[pnuu]

Thanks for the review, I'll have a closer look tomorrow of the two remaining comments when I'm at the work laptop and a bigger screen.

I went with contrast as that's what all the functions in the module adjust. Well, it's debatable if invert() does that...

[pnuu]

I moved trollimage.colormap.Colormap imports to the functions using it in enhancements/colormap.py. Yeah, I know that it is expected it is imported at the module level seeing the name of the module, but the lookup() enhancement doesn't need it so thought it might help in some very edgy use-cases to reduce the footprint.

[pnuu]

Fixed the conflicts from https://github.com/pytroll/satpy/commit/562ac1ebd580550ba93c46e6eb438cc9e273632e