Fix true colors generation for AHI HSD data and refactor the dep tree code

[mraspaud]

Refactor the dep tree code and fix the true color composite for instruments with channels at different resolutions.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

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[coveralls]

Coverage increased (+0.2%) to 90.241% when pulling 9fdbd7a on mraspaud:fix-true-color-ahi into cdc8ce5 on pytroll:master.

[codecov]

Codecov Report

Merging #1313 into master will increase coverage by 0.17%.
The diff coverage is 97.46%.

@@            Coverage Diff             @@
##           master    #1313      +/-   ##
==========================================
+ Coverage   90.06%   90.24%   +0.17%     
==========================================
  Files         220      222       +2     
  Lines       32419    32719     +300     
==========================================
+ Hits        29198    29526     +328     
+ Misses       3221     3193      -28     

Impacted Files	Coverage Δ
satpy/__init__.py	86.66% <ø> (+0.95%)	⬆️
satpy/tests/utils.py	95.40% <ø> (ø)
satpy/dataset.py	93.07% <91.30%> (+0.14%)	⬆️
satpy/dependency_tree.py	97.26% <97.26%> (ø)
satpy/node.py	86.36% <100.00%> (-8.12%)	⬇️
satpy/readers/yaml_reader.py	95.23% <100.00%> (ø)
satpy/scene.py	90.58% <100.00%> (+0.06%)	⬆️
satpy/tests/test_dataset.py	100.00% <100.00%> (ø)
satpy/tests/test_dependency_tree.py	100.00% <100.00%> (ø)
satpy/tests/test_node.py	100.00% <100.00%> (ø)
... and 10 more

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[djhoese]

With this transitive change on resolution you'll need to update the dev guide docs which use resolution as an example.

I also don't use transitive in my everyday vocabulary (I don't have a very large one) but I'm confused at which definition of transitive fits here with how it is being used.

[mraspaud]

The idea is that if the compositor C needs dependency D1 with property P with value V, and D1 depends on D2, then D2 should have property P with value V also (if applicable). Indeed, this isn't strict transitivity, but I couldn't come up with a better name for it. Suggestions are welcome.

[djhoese]

I don't see anything wrong with this. I'm sure there will be surprises, but better to have more things working and getting towards a fully working redesign/refactor. I had one comment about the new exception but otherwise I'll just ask for a self-review of the new methods you added to everything and make sure that they actually need to be public. Otherwise LGTM.

[mraspaud]

I think things are in order with public/private