Add the reinhard enhancements

[mraspaud]

This PR adds a reinhard-based tonemapping enhancement.

• Tests added
• Fully documented

An example on an Himawari 8 True Color image

Reinhard:

Cira stretch

[codecov]

Codecov Report

Merging #1623 (de1099a) into master (f6fe88f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1623   +/-   ##
=======================================
  Coverage   92.60%   92.60%           
=======================================
  Files         258      258           
  Lines       37716    37739   +23     
=======================================
+ Hits        34927    34950   +23     
  Misses       2789     2789           

Flag	Coverage Δ
behaviourtests	4.81% <13.04%> (+<0.01%)	⬆️
unittests	92.74% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/enhancements/__init__.py	97.35% <100.00%> (+0.29%)	⬆️
satpy/tests/enhancement_tests/test_enhancements.py	100.00% <100.00%> (ø)

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[djhoese]

Looks good to me. Should it be documented somewhere? I suppose we don't really have a lot of enhancement documentation.

Also, I'm wondering if this should be wrapped in a single function that gets with map_blocks? We've seen in other areas that limiting the number of tasks that dask has to deal with can improve the performance. Just an idea, not a requirement for merge.

[mraspaud]

I'm going to merge this as it is because:

I think if we want to limit the number of dask tasks, it should be done at a higher level, for example in the code that calls the enhancements. Otherwise we will end up with many map_blocks everywhere that will lower readability and maintenability I think.

[djhoese]

That was the intention (at least a little bit) with the apply_enhancement function that is in enhancements/__init__.py.