Fix DifferenceCompositor ignoring YAML metadata

[djhoese]

I needed to be able to set a standard_name for my difference and noticed that it was being ignored. This pull request fixes that.

• Tests added

[codecov]

Codecov Report

Merging #1564 (4c987f0) into master (d556c80) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1564      +/-   ##
==========================================
- Coverage   92.54%   92.54%   -0.01%     
==========================================
  Files         251      251              
  Lines       36761    36763       +2     
==========================================
+ Hits        34022    34023       +1     
- Misses       2739     2740       +1     

Flag	Coverage Δ
behaviourtests	4.49% <25.00%> (-0.01%)	⬇️
unittests	92.68% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
satpy/composites/__init__.py	88.28% <100.00%> (+0.01%)	⬆️
satpy/tests/test_composites.py	99.87% <100.00%> (+<0.01%)	⬆️
satpy/scene.py	92.07% <0.00%> (-0.18%)	⬇️

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[djhoese]

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