Fix delayed generation of composites and composite resolution

[mraspaud]

This PR fixes three bugs in the composite generation part of satpy.

1. The empty_node is now a class attribute to make sure it is the same across multiple instances of the dependency tree. This fixes a bug appearing when the composite generation is delayed, which causes the dependency tree to be copied and previously a net empty_node to be created.
2. The resolution of a composite made of datasets with the same resolution is now set (it was None).
3. Two composites with the same dependencies, loaded one after the other with different resolutions are now loaded with the proper respective resolution

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

[codecov]

Codecov Report

Merging #828 into master will increase coverage by 0.04%.
The diff coverage is 97.33%.

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
+ Coverage    83.9%   83.95%   +0.04%     
==========================================
  Files         167      167              
  Lines       24445    24512      +67     
==========================================
+ Hits        20511    20578      +67     
  Misses       3934     3934

Impacted Files	Coverage Δ
satpy/tests/utils.py	97.08% <100%> (ø)	⬆️
satpy/tests/compositor_tests/__init__.py	99.53% <100%> (ø)	⬆️
satpy/node.py	94.79% <100%> (+0.28%)	⬆️
satpy/tests/test_scene.py	99.48% <100%> (+0.01%)	⬆️
satpy/composites/__init__.py	74.74% <60%> (+0.09%)	⬆️

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[coveralls]

Coverage increased (+0.04%) to 83.926% when pulling 3301cf2 on mraspaud:fix-modis-processing into 2654cf3 on pytroll:master.

[djhoese]

LGTM. Would it be possible to add some tests for this?

[mraspaud]

Good question :)

[mraspaud]

Test added

[djhoese]

How hard would it be to add a test for the delayed generation "empty" node stuff? I suppose that would be with the tests I usually write for reading. We must have one for delayed generation already.

[mraspaud]

I think this is ready for re-review. I'd like to have it included in the patch release if possible.

[djhoese]

There are a couple things that could be cleaned up, but otherwise it looks good to me. I had one question about one of the tests.

Nice job deciphering my Scene/reader tests and adding your own.

[djhoese]

Travis notification is stuck, but all tests pass. Merging now.