Add image 'mode' as identifying attribute to enhancements

[djhoese]

Similar to how we have name and standard_name as identifying factors that someone can use in the enhancement configs, this PR adds mode. This is used in the new RGB default enhancement which does a crude linear stretch between 0 and 1. This is a nice thing to have when creating new RGBs and experimenting with them because we don't have to configure a new enhancement every time. This is also useful for DayNightCompositor composites which are already enhanced and on a 0 to 1 scale. This way you could make any number of DayNight composites and wouldn't need to make a new 0 to 1 enhancement for each one.

I should really add documentation for this and it needs tests but I wanted to have people look at this as soon as possible since I'd like this for my scipy tutorial.

The other thing that is not implemented here but has been talked about is the shortcut of allowing certain attributes to "shortcut" the enhancement process. So if an RGB is detected (via the enhancement mode) then a special RGB-friendly enhancement would be used which would default to color limits of 0 and 1 for each band but could also use a color_limits attribute or vmin and vmax attributes if they existed to override the 0 and 1. I'm not sure this is the right time to implement this functionality since it seems like a work around for the general problem of "creating enhancements is more annoying than it should be for simple composites".

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented

[codecov]

Codecov Report

Merging #817 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
+ Coverage   83.72%   83.75%   +0.02%     
==========================================
  Files         165      165              
  Lines       24115    24167      +52     
==========================================
+ Hits        20190    20240      +50     
- Misses       3925     3927       +2

Impacted Files	Coverage Δ
satpy/writers/__init__.py	85.64% <100%> (+0.43%)	⬆️
satpy/tests/test_writers.py	97.89% <100%> (+0.07%)	⬆️
satpy/tests/compositor_tests/__init__.py	99.52% <100%> (ø)	⬆️
satpy/enhancements/__init__.py	85.89% <100%> (+0.66%)	⬆️
satpy/composites/__init__.py	74.39% <0%> (+0.2%)	⬆️

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[coveralls]

Coverage increased (+0.04%) to 83.746% when pulling aefe7a4 on djhoese:feature-rgb-enhancer into 5b74266 on pytroll:master.

[gerritholl]

@djhoese What is the status of this PR? Is this still current? Seems to have some merge conflicts.

[djhoese]

@gerritholl I still like the idea of this PR but I think @pnuu and @mraspaud didn't like it so much. I can fix the conflicts, but am curious what their current opinion is.